Method and device for updating system information in wireless LAN system

ABSTRACT

The present invention relates to a method and a device for updating system information in a wireless LAN system. A method for updating system information in a station (STA) of a wireless communication system may comprise transmitting, by the STA which stores system information and a configuration change count value of a previously linked preferred access point (AP), a probe request frame for active scanning to the preferred AP; and receiving a probe response frame from the preferred AP. Preferably, the probe request frame includes a configuration change count field previously acquired from the preferred AP, and if a value of the configuration change count field included in the probe request frame is different from a present configuration change count value of the preferred AP, the probe response frame may include one or more elements of system information which should be updated by the STA.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage filing under 35 U.S.C. 371 ofInternational Application No. PCT/KR2013/007301, filed on Aug. 13, 2013,which claims the benefit of U.S. Provisional Application Ser. No.61/682,326, filed on Aug. 13, 2012, 61/694,263, filed on Aug. 29, 2012,61/702,259, filed on Sep. 18, 2012, 61/703,214, filed on Sep. 19, 2012,61/712,286, filed on Oct. 11, 2012, and 61/857,684, filed on Jul. 23,2013, the contents of which are all hereby incorporated by referenceherein in their entirety.

TECHNICAL FIELD

The present invention relates to a wireless communication system and,more specifically, to a method and a device for updating systeminformation in a wireless LAN system.

BACKGROUND ART

Various wireless communication technologies systems have been developedwith rapid development of information communication technologies. WLANtechnology from among wireless communication technologies allowswireless Internet access at home or in enterprises or at a specificservice provision region using mobile terminals, such as a PersonalDigital Assistant (PDA), a laptop computer, a Portable Multimedia Player(PMP), etc. on the basis of Radio Frequency (RF) technology.

In order to obviate limited communication speed, one of the advantagesof WLAN, the recent technical standard has proposed an evolved systemcapable of increasing the speed and reliability of a network whilesimultaneously extending a coverage region of a wireless network. Forexample, IEEE 802.11n enables a data processing speed to support amaximum high throughput (HT) of 540 Mbps. In addition, Multiple Inputand Multiple Output (MIMO) technology has recently been applied to botha transmitter and a receiver so as to minimize transmission errors aswell as to optimize a data transfer rate.

DISCLOSURE Technical Problem

M2M (Machine-to-Machine) communication technology is discussed asnext-generation communication technology. In IEEE 802.11 WLAN systems,IEEE 802.11ah is being developed as a technical standard for supportingM2M communication. A scenario in which a small quantity of data istransmitted and received at a low speed occasionally in an environmenthaving so many devices may be considered in M2M communication.

Communication in wireless LAN systems is performed through a mediumshared by all devices. When the number of devices increases as in M2Mcommunication, taking a long time for channel access of one device cannot only cause system performance deterioration but also obstruct powersaving of devices.

An object of the present invention is to provide a new mechanism ofupdating system information.

The technical problems solved by the present invention are not limitedto the above technical problems and those skilled in the art mayunderstand other technical problems from the following description.

Technical Solution

To accomplish the object of the present invention, there is provided amethod for updating system information in a station (STA) of a wirelesscommunication system, including: transmitting, by the STA, a proberequest frame for active scanning to a previously associated preferredaccess point (AP), the STA storing system information and aconfiguration change count value of the previously associated preferredAP; and receiving a probe response frame from the preferred AP. Theprobe request frame includes a configuration change count fieldpreviously acquired from the preferred AP, and when a value of theconfiguration change count field included in the probe request frame isdifferent from a current configuration change count value of thepreferred AP, the probe response frame includes one or more elements ofsystem information required to be updated by the STA.

To accomplish the object of the present invention, there is provided amethod for providing system information updated in an AP of a wirelesscommunication system, including: receiving a probe request frame foractive scanning from a preferred STA previously associated with the APand storing system information and a configuration change count value ofthe AP; and transmitting a probe response frame to the preferred STA.The probe request frame includes a configuration change count fieldpreviously acquired by the preferred STA from the AP, and when a valueof the configuration change count field included in the probe requestframe is different from a current configuration change count value ofthe AP, the probe response frame includes one or more elements of systeminformation required to be updated by the preferred STA.

To accomplish the object of the present invention, there is provided astation (STA) updating system information in a wireless communicationsystem, including: a memory storing system information and aconfiguration change count value of a previously associated preferredAP; a transceiver; and a processor configured to transmit a proberequest frame for active scanning to the preferred AP and to receive aprobe response frame from the AP. The probe request frame includes aconfiguration change count field previously acquired from the preferredAP, and when a value of the configuration change count field included inthe probe request frame is different from a current configuration changecount value of the preferred AP, the probe response frame includes oneor more elements of system information required to be updated by theSTA.

To accomplish the object of the present invention, there is provided anAP providing updated system information in a wireless communicationsystem, including: a transceiver; and a processor configured to receivea probe request frame for active scanning from a preferred STApreviously associated with the AP and storing system information and aconfiguration change count value of the AP and to transmit a proberesponse frame to the preferred STA. The probe request frame includes aconfiguration change count field previously acquired by the preferredSTA from the AP, and when a value of the configuration change countfield included in the probe request frame is different from a currentconfiguration change count value of the AP, the probe response frameincludes one or more elements of system information required to beupdated by the preferred STA.

The above description and the following description are exemplary andare for additional explanation of the claims.

Advantageous Effects

The present invention can provide a new apparatus and method forupdating system information.

It will be appreciated by persons skilled in the art that that theeffects that can be achieved through the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIG. 1 exemplarily shows an IEEE 802.11 system according to oneembodiment of the present invention.

FIG. 2 exemplarily shows an IEEE 802.11 system according to anotherembodiment of the present invention.

FIG. 3 exemplarily shows an IEEE 802.11 system according to stillanother embodiment of the present invention.

FIG. 4 is a conceptual diagram illustrating a WLAN system.

FIG. 5 is a flowchart illustrating a link setup process for use in theWLAN system.

FIG. 6 is a conceptual diagram illustrating a backoff process.

FIG. 7 is a conceptual diagram illustrating a hidden node and an exposednode.

FIG. 8 is a conceptual diagram illustrating RTS (Request To Send) andCTS (Clear To Send).

FIG. 9 is a conceptual diagram illustrating a power managementoperation.

FIGS. 10 to 12 are conceptual diagrams illustrating detailed operationsof a station (STA) having received a Traffic Indication Map (TIM).

FIG. 13 is a conceptual diagram illustrating a group-based AID.

FIG. 14 is a conceptual diagram illustrating a short beacon.

FIG. 15 is a conceptual diagram illustrating exemplary fields includedin a short beacon frame.

FIG. 16 illustrates a short beacon frame format according to anembodiment of the present invention.

FIG. 17 illustrates a short beacon frame format according to anotherembodiment of the present invention.

FIG. 18 is a diagram illustrating a method for transmitting andreceiving a full beacon frame according to an embodiment of the presentinvention.

FIG. 19 is a diagram illustrating a method for transmitting andreceiving a full beacon frame according to another embodiment of thepresent invention.

FIG. 20 is a diagram illustrating a method for transmitting andreceiving a full beacon frame according to another embodiment of thepresent invention.

FIG. 21 is a diagram illustrating transmission of a probe response framein a broadcast manner.

FIG. 22 illustrates a change sequence field.

FIG. 23 is a diagram illustrating a probe request/response procedureaccording to an embodiment of the present invention.

FIG. 24 is a diagram illustrating a probe request/response procedureaccording to another embodiment of the present invention.

FIG. 25 is a diagram illustrating a probe request/response procedureaccording to another embodiment of the present invention.

FIG. 26 is a diagram illustrating an SI update request/responseprocedure according to an embodiment of the present invention.

FIG. 27 is a diagram illustrating a method for updating systeminformation using a full beacon request frame.

FIG. 28 is a diagram illustrating an example of performing fast initiallink setup during active scanning.

FIG. 29 is a diagram illustrating an example of performing fast initiallink setup during passive scanning.

FIG. 30 is a diagram illustrating a procedure of setting an associatedAP as a preferred AP.

FIG. 31 illustrates an operation when active scanning is performed for apreferred AP which was dissociated.

FIG. 32 illustrates an exemplary FILS probe request frame.

FIG. 33 illustrates an exemplary FILS probe request frame having a shortMAC header applied thereto.

FIG. 34 illustrates a short MAC header.

FIG. 35 illustrates another exemplary short MAC header.

FIG. 36 illustrates another exemplary FILS probe request frame.

FIG. 37 illustrates an exemplary FILS probe response frame.

FIG. 38 is a diagram illustrating a system information updaterequest/response procedure according to an embodiment of the presentinvention.

FIG. 39 illustrates an example of transmitting a probe response frame ina unicast manner.

FIG. 40 illustrates an example of transmitting a probe response frame ina broadcast manner.

FIG. 41 is a diagram illustrating an example in which an FILS responseframe includes a duration to next full beacon field or information onthe next TBTT.

FIG. 42 is a diagram illustrating an example in which an FILS responseframe includes information for requesting transmission of a normal proberequest frame.

FIG. 43 is a block diagram illustrating a configuration of a radioapparatus according to an embodiment of the present invention.

BEST MODE

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. The detailed description, which will be given below withreference to the accompanying drawings, is intended to explain exemplaryembodiments of the present invention, rather than to show the onlyembodiments that can be implemented according to the present invention.The following detailed description includes specific details in order toprovide a thorough understanding of the present invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced without such specific details.

The following embodiments are proposed by combining constituentcomponents and characteristics of the present invention according to apredetermined format. The individual constituent components orcharacteristics should be considered optional factors on the conditionthat there is no additional remark. If required, the individualconstituent components or characteristics may not be combined with othercomponents or characteristics. In addition, some constituent componentsand/or characteristics may be combined to implement the embodiments ofthe present invention. The order of operations to be disclosed in theembodiments of the present invention may be changed. Some components orcharacteristics of any embodiment may also be included in otherembodiments, or may be replaced with those of the other embodiments asnecessary.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention andimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an Institute of Electrical and Electronics Engineers (IEEE)802 system, a 3rd Generation Partnership Project (3GPP) system, a 3GPPLong Term Evolution (LTE) system, an LTE-Advanced (LTE-A) system, and a3GPP2 system. In particular, steps or parts, which are not described toclearly reveal the technical idea of the present invention, in theembodiments of the present invention may be supported by the abovedocuments. All terminology used herein may be supported by at least oneof the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, CDMA (CodeDivision Multiple Access), FDMA (Frequency Division Multiple Access),TDMA (Time Division Multiple Access), OFDMA (Orthogonal FrequencyDivision Multiple Access), SC-FDMA (Single Carrier Frequency DivisionMultiple Access), and the like. CDMA may be embodied through wireless(or radio) technology such as UTRA (Universal Terrestrial Radio Access)or CDMA2000. TDMA may be embodied through wireless (or radio) technologysuch as GSM (Global System for Mobile communication)/GPRS (GeneralPacket Radio Service)/EDGE (Enhanced Data Rates for GSM Evolution).OFDMA may be embodied through wireless (or radio) technology such asInstitute of Electrical and Electronics Engineers (IEEE) 802.11 (Wi-Fi),IEEE 802.16 (WiMAX), IEEE 802-20, and E-UTRA (Evolved UTRA). Forclarity, the following description focuses on IEEE 802.11 systems.However, technical features of the present invention are not limitedthereto.

WLAN System Structure

FIG. 1 exemplarily shows an IEEE 802.11 system according to oneembodiment of the present invention.

The structure of the IEEE 802.11 system may include a plurality ofcomponents. A WLAN which supports transparent STA mobility for a higherlayer may be provided by mutual operations of the components. A BasicService Set (BSS) may correspond to a basic constituent block in an IEEE802.11 LAN. In FIG. 1, two BSSs (BSS1 and BSS2) are shown and two STAsare included in each of the BSSs (i.e. STA1 and STA2 are included inBSS1 and STA3 and STA4 are included in BSS2). An ellipse indicating theBSS in FIG. 1 may be understood as a coverage area in which STAsincluded in the corresponding BSS maintain communication. This area maybe referred to as a Basic Service Area (BSA). If an STA moves out of theBSA, the STA cannot directly communicate with the other STAs in thecorresponding BSA.

In the IEEE 802.11 LAN, the most basic type of BSS is an Independent BSS(IBSS). For example, the IBSS may have a minimum form consisting of onlytwo STAs. The BSS (BSS1 or BSS2) of FIG. 1, which is the simplest formand in which other components are omitted, may correspond to a typicalexample of the IBSS. Such configuration is possible when STAs candirectly communicate with each other. Such a type of LAN is notprescheduled and may be configured when the LAN is necessary. This maybe referred to as an ad-hoc network.

Memberships of an STA in the BSS may be dynamically changed when the STAis switched on or off or the STA enters or leaves the BSS region. TheSTA may use a synchronization process to join the BSS. To access allservices of a BSS infrastructure, the STA should be associated with theBSS. Such association may be dynamically configured and may include useof a Distribution System Service (DSS).

FIG. 2 is a diagram showing another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In FIG. 2,components such as a Distribution System (DS), a Distribution SystemMedium (DSM), and an Access Point (AP) are added to the structure ofFIG. 1.

A direct STA-to-STA distance in a LAN may be restricted by PHYperformance. In some cases, such restriction of the distance may besufficient for communication. However, in other cases, communicationbetween STAs over a long distance may be necessary. The DS may beconfigured to support extended coverage.

The DS refers to a structure in which BSSs are connected to each other.Specifically, a BSS may be configured as a component of an extended formof a network consisting of a plurality of BSSs, instead of independentconfiguration as shown in FIG. 1.

The DS is a logical concept and may be specified by the characteristicof the DSM. In relation to this, a Wireless Medium (WM) and the DSM arelogically distinguished in IEEE 802.11. Respective logical media areused for different purposes and are used by different components. Indefinition of IEEE 802.11, such media are not restricted to the same ordifferent media. The flexibility of the IEEE 802.11 LAN architecture (DSarchitecture or other network architectures) can be explained in that aplurality of media is logically different. That is, the IEEE 802.11 LANarchitecture can be variously implemented and may be independentlyspecified by a physical characteristic of each implementation.

The DS may support mobile devices by providing seamless integration ofmultiple BSSs and providing logical services necessary for handling anaddress to a destination.

The AP refers to an entity that enables associated STAs to access the DSthrough a WM and that has STA functionality. Data may move between theBSS and the DS through the AP. For example, STA2 and STA3 shown in FIG.2 have STA functionality and provide a function of causing associatedSTAs (STA1 and STA4) to access the DS. Moreover, since all APscorrespond basically to STAs, all APs are addressable entities. Anaddress used by an AP for communication on the WM need not always beidentical to an address used by the AP for communication on the DSM.

Data transmitted from one of STAs associated with the AP to an STAaddress of the AP may always be received by an uncontrolled port and maybe processed by an IEEE 802.1X port access entity. If the controlledport is authenticated, transmission data (or frame) may be transmittedto the DS.

FIG. 3 is a diagram showing still another exemplary structure of an IEEE802.11 system to which the present invention is applicable. In additionto the structure of FIG. 2, FIG. 3 conceptually shows an ExtendedService Set (ESS) for providing wide coverage.

A wireless network having arbitrary size and complexity may be comprisedof a DS and BSSs. In the IEEE 802.11 system, such a type of network isreferred to an ESS network. The ESS may correspond to a set of BSSsconnected to one DS. However, the ESS does not include the DS. The ESSnetwork is characterized in that the ESS network appears as an IBSSnetwork in a Logical Link Control (LLC) layer. STAs included in the ESSmay communicate with each other and mobile STAs are movabletransparently in LLC from one BSS to another BSS (within the same ESS).

In IEEE 802.11, relative physical locations of the BSSs in FIG. 3 arenot assumed and the following forms are all possible. BSSs may partiallyoverlap and this form is generally used to provide continuous coverage.BSSs may not be physically connected and the logical distances betweenBSSs have no limit. BSSs may be located at the same physical positionand this form may be used to provide redundancy. One or more IBSSs orESS networks may be physically located in the same space as one or moreESS networks. This may correspond to an ESS network form in the case inwhich an ad-hoc network operates in a location in which an ESS networkis present, the case in which IEEE 802.11 networks of differentorganizations physically overlap, or the case in which two or moredifferent access and security policies are necessary in the samelocation.

FIG. 4 is a diagram showing an exemplary structure of a WLAN system. InFIG. 4, an example of an infrastructure BSS including a DS is shown.

In the example of FIG. 4, BSS1 and BSS2 constitute an ESS. In the WLANsystem, an STA is a device operating according to MAC/PHY regulation ofIEEE 802.11. STAs include AP STAs and non-AP STAs. The non-AP STAscorrespond to devices, such as laptop computers or mobile phones,handled directly by users. In FIG. 4, STA1, STA3, and STA4 correspond tothe non-AP STAs and STA2 and STA5 correspond to AP STAs.

In the following description, the non-AP STA may be referred to as aterminal, a Wireless Transmit/Receive Unit (WTRU), a User Equipment(UE), a Mobile Station (MS), a mobile terminal, or a Mobile SubscriberStation (MSS). The AP is a concept corresponding to a Base Station (BS),a Node-B, an evolved Node-B (e-NB), a Base Transceiver System (BTS), ora femto BS in other wireless communication fields.

Link Setup Process

FIG. 5 is a flowchart explaining a general link setup process accordingto an exemplary embodiment of the present invention.

In order to allow an STA to establish link setup on the network as wellas to transmit/receive data over the network, the STA must perform suchlink setup through processes of network discovery, authentication, andassociation, and must establish association and perform securityauthentication. The link setup process may also be referred to as asession initiation process or a session setup process. In addition, anassociation step is a generic term for discovery, authentication,association, and security setup steps of the link setup process.

Link setup process is described referring to FIG. 5.

In step S510, STA may perform the network discovery action. The networkdiscovery action may include the STA scanning action. That is, STA mustsearch for an available network so as to access the network. The STAmust identify a compatible network before participating in a wirelessnetwork. Here, the process for identifying the network contained in aspecific region is referred to as a scanning process.

The scanning scheme is classified into active scanning and passivescanning.

FIG. 5 is a flowchart illustrating a network discovery action includingan active scanning process. In the case of the active scanning, an STAconfigured to perform scanning transmits a probe request frame and waitsfor a response to the probe request frame, such that the STA can movebetween channels and at the same time can determine which AP (AccessPoint) is present in a peripheral region. A responder transmits a proberesponse frame, acting as a response to the probe request frame, to theSTA having transmitted the probe request frame. In this case, theresponder may be an STA that has finally transmitted a beacon frame in aBSS of the scanned channel. In BSS, since the AP transmits the beaconframe, the AP operates as a responder. In IBSS, since STAs of the IBSSsequentially transmit the beacon frame, the responder is not constant.For example, the STA, that has transmitted the probe request frame atChannel #1 and has received the probe response frame at Channel #1,stores BSS-associated information contained in the received proberesponse frame, and moves to the next channel (for example, Channel #2),such that the STA may perform scanning using the same method (i.e.,probe request/response transmission/reception at Channel #2).

Although not shown in FIG. 5, the scanning action may also be carriedout using passive scanning. An STA configured to perform scanning in thepassive scanning mode waits for a beacon frame while simultaneouslymoving from one channel to another channel. The beacon frame is one ofmanagement frames in IEEE 802.11, indicates the presence of a wirelessnetwork, enables the STA performing scanning to search for the wirelessnetwork, and is periodically transmitted in a manner that the STA canparticipate in the wireless network. In BSS, the AP is configured toperiodically transmit the beacon frame. In IBSS, STAs of the IBSS areconfigured to sequentially transmit the beacon frame. If each STA forscanning receives the beacon frame, the STA stores BSS informationcontained in the beacon frame, and moves to another channel and recordsbeacon frame information at each channel. The STA having received thebeacon frame stores BSS-associated information contained in the receivedbeacon frame, moves to the next channel, and thus performs scanningusing the same method.

In comparison between the active scanning and the passive scanning, theactive scanning is more advantageous than the passive scanning in termsof delay and power consumption.

After the STA discovers the network, the STA may perform theauthentication process in step S520. The authentication process may bereferred to as a first authentication process in such a manner that theauthentication process can be clearly distinguished from the securitysetup process of step S540.

The authentication process may include transmitting an authenticationrequest frame to an AP by the STA, and transmitting an authenticationresponse frame to the STA by the AP in response to the authenticationrequest frame. The authentication frame used for authenticationrequest/response may correspond to a management frame.

The authentication frame may include an authentication algorithm number,an authentication transaction sequence number, a state code, a challengetext, a Robust Security Network (RSN), a Finite Cyclic Group (FCG), etc.The above-mentioned information contained in the authentication framemay correspond to some parts of information capable of being containedin the authentication request/response frame, may be replaced with otherinformation, or may include additional information.

The STA may transmit the authentication request frame to the AP. The APmay decide whether to authenticate the corresponding STA on the basis ofinformation contained in the received authentication request frame. TheAP may provide the authentication result to the STA through theauthentication response frame.

After the STA has been successfully authenticated, the associationprocess may be carried out in step S530. The association process mayinvolve transmitting an association request frame to the AP by the STA,and transmitting an association response frame to the STA by the AP inresponse to the association request frame.

For example, the association request frame may include informationassociated with various capabilities, a beacon listen interval, aService Set Identifier (SSID), supported rates, supported channels, RSN,mobility domain, supported operating classes, a TIM (Traffic IndicationMap) broadcast request, interworking service capability, etc.

For example, the association response frame may include informationassociated with various capabilities, a state code, an Association ID(AID), supported rates, an Enhanced Distributed Channel Access (EDCA)parameter set, a Received Channel Power Indicator (RCPI), a ReceivedSignal to Noise Indicator (RSNI), mobility domain, a timeout interval(association comeback time), an overlapping BSS scan parameter, a TIMbroadcast response, a QoS map, etc.

The above-mentioned information may correspond to some parts ofinformation capable of being contained in the associationrequest/response frame, may be replaced with other information, or mayinclude additional information.

After the STA has been successfully associated with the network, asecurity setup process may be carried out in step S540. The securitysetup process of Step S540 may be referred to as an authenticationprocess based on Robust Security Network Association (RSNA)request/response. The authentication process of step S520 may bereferred to as a first authentication process, and the security setupprocess of Step S540 may also be simply referred to as an authenticationprocess.

For example, the security setup process of Step S540 may include aprivate key setup process through 4-way handshaking based on an(Extensible Authentication Protocol over LAN (EAPOL) frame. In addition,the security setup process may also be carried out according to othersecurity schemes not defined in IEEE 802.11 standards.

WLAN Evolution

In order to obviate limitations in WLAN communication speed, IEEE802.11n has recently been established as a communication standard. IEEE802.11n aims to increase network speed and reliability as well as toextend a coverage region of the wireless network. In more detail, IEEE802.11n supports a High Throughput (HT) of a maximum of 540 Mbps, and isbased on MIMO technology in which multiple antennas are mounted to eachof a transmitter and a receiver.

With the widespread use of WLAN technology and diversification of WLANapplications, there is a need to develop a new WLAN system capable ofsupporting a HT higher than a data processing speed supported by IEEE802.11n. The next generation WLAN system for supporting Very HighThroughput (VHT) is the next version (for example, IEEE 802.11ac) of theIEEE 802.11n WLAN system, and is one of IEEE 802.11 WLAN systemsrecently proposed to support a data process speed of 1 Gbps or more at aMAC SAP (Medium Access Control Service Access Point).

In order to efficiently utilize a radio frequency (RF) channel, the nextgeneration WLAN system supports MU-MIMO (Multi User Multiple InputMultiple Output) transmission in which a plurality of STAs cansimultaneously access a channel. In accordance with the MU-MIMOtransmission scheme, the AP may simultaneously transmit packets to atleast one MIMO-paired STA.

In addition, a technology for supporting WLAN system operations inwhitespace has recently been discussed. For example, a technology forintroducing the WLAN system in whitespace (TV WS) such as an idlefrequency band (for example, 54˜698 MHz band) left because of thetransition to digital TV has been discussed under the IEEE 802.11afstandard. However, the above-mentioned information is disclosed forillustrative purposes only, and the whitespace may be a licensed bandcapable of being primarily used only by a licensed user. The licenseduser may be a user who has authority to use the licensed band, and mayalso be referred to as a licensed device, a primary user, an incumbentuser, or the like.

For example, an AP and/or STA operating in the whitespace (WS) mustprovide a function for protecting the licensed user. For example,assuming that the licensed user such as a microphone has already used aspecific WS channel acting as a divided frequency band on regulation ina manner that a specific bandwidth is occupied from the WS band, the APand/or STA cannot use the frequency band corresponding to thecorresponding WS channel so as to protect the licensed user. Inaddition, the AP and/or STA must stop using the corresponding frequencyband under the condition that the licensed user uses a frequency bandused for transmission and/or reception of a current frame.

Therefore, the AP and/or STA must determine whether to use a specificfrequency band of the WS band. In other words, the AP and/or STA mustdetermine the presence or absence of an incumbent user or a licenseduser in the frequency band. The scheme for determining the presence orabsence of the incumbent user in a specific frequency band is referredto as a spectrum sensing scheme. An energy detection scheme, a signaturedetection scheme and the like may be used as the spectrum sensingmechanism. The AP and/or STA may determine that the frequency band isbeing used by an incumbent user if the intensity of a received signalexceeds a predetermined value, or when a DTV preamble is detected.

M2M (Machine to Machine) communication technology has been discussed asnext generation communication technology. Technical standard forsupporting M2M communication has been developed as IEEE 802.11ah in theIEEE 802.11 WLAN system. M2M communication refers to a communicationscheme including one or more machines, or may also be referred to asMachine Type Communication (MTC) or Machine To Machine (M2M)communication. In this case, the machine may be an entity that does notrequire direct handling and intervention of a user. For example, notonly a meter or vending machine including a RF module, but also a userequipment (UE) (such as a smartphone) capable of performingcommunication by automatically accessing the network without userintervention/handling may be an example of such machines. M2Mcommunication may include Device-to-Device (D2D) communication andcommunication between a device and an application server, etc. Asexemplary communication between the device and the application server,communication between a vending machine and an application server,communication between the Point of Sale (POS) device and the applicationserver, and communication between an electric meter, a gas meter or awater meter and the application server. M2M-based communicationapplications may include security, transportation, healthcare, etc. Inthe case of considering the above-mentioned application examples, M2Mcommunication has to support the method for sometimestransmitting/receiving a small amount of data at low speed under anenvironment including a large number of devices.

In more detail, M2M communication must support a large number of STAs.Although the current WLAN system assumes that one AP is associated witha maximum of 2007 STAs, various methods for supporting other cases inwhich many more STAs (e.g., about 6000 STAs) are associated with one APhave recently been discussed in M2M communication. In addition, it isexpected that many applications for supporting/requesting a low transferrate are present in M2M communication. In order to smoothly support manySTAs, the WLAN system may recognize the presence or absence of data tobe transmitted to the STA on the basis of a TIM (Traffic Indicationmap), and various methods for reducing the bitmap size of the TIM haverecently been discussed. In addition, it is expected that much trafficdata having a very long transmission/reception interval is present inM2M communication. For example, in M2M communication, a very smallamount of data (e.g., electric/gas/water metering) needs to betransmitted at long intervals (for example, every month). Therefore,although the number of STAs associated with one AP increases in the WLANsystem, many developers and companies are conducting intensive researchinto an WLAN system which can efficiently support the case in whichthere are a very small number of STAs, each of which has a data frame tobe received from the AP during one beacon period.

As described above, WLAN technology is rapidly developing, and not onlythe above-mentioned exemplary technologies but also other technologiessuch as a direct link setup, improvement of media streaming throughput,high-speed and/or support of large-scale initial session setup, andsupport of extended bandwidth and operation frequency, are beingintensively developed.

Medium Access Mechanism

In the IEEE 802.11-based WLAN system, a basic access mechanism of MAC(Medium Access Control) is a Carrier Sense Multiple Access withCollision Avoidance (CSMA/CA) mechanism. The CSMA/CA mechanism isreferred to as a Distributed Coordination Function (DCF) of IEEE 802.11MAC, and basically includes a “Listen Before Talk” access mechanism. Inaccordance with the above-mentioned access mechanism, the AP and/or STAmay perform Clear Channel Assessment (CCA) for sensing an RF channel ormedium during a predetermined time interval [for example, DCFInter-Frame Space (DIFS)], prior to data transmission. If it isdetermined that the medium is in the idle state, frame transmissionthrough the corresponding medium begins. On the other hand, if it isdetermined that the medium is in the occupied state, the correspondingAP and/or STA does not start its own transmission, establishes a delaytime (for example, a random backoff period) for medium access, andattempts to start frame transmission after waiting for a predeterminedtime. Through application of a random backoff period, it is expectedthat multiple STAs will attempt to start frame transmission afterwaiting for different times, resulting in minimum collision.

In addition, IEEE 802.11 MAC protocol provides a Hybrid CoordinationFunction (HCF). HCF is based on DCF and Point Coordination Function(PCF). PCF refers to the polling-based synchronous access scheme inwhich periodic polling is executed in a manner that all reception (Rx)APs and/or STAs can receive the data frame. In addition, HCF includesEnhanced Distributed Channel Access (EDCA) and HCF Controlled ChannelAccess (HCCA). EDCA is achieved when the access scheme provided from aprovider to a plurality of users is contention-based. HCCA is achievedby the contention-free-based channel access scheme based on the pollingmechanism. In addition, HCF includes a medium access mechanism forimproving Quality of Service (QoS) of WLAN, and may transmit QoS data inboth a Contention Period (CP) and a Contention Free Period (CFP).

FIG. 6 is a conceptual diagram illustrating a backoff process.

Operations based on a random backoff period will hereinafter bedescribed with reference to FIG. 6. If the occupy- or busy-state mediumis shifted to an idle state, several STAs may attempt to transmit data(or frame). As a method for implementing a minimum number of collisions,each STA selects a random backoff count, waits for a slot timecorresponding to the selected backoff count, and then attempts to startdata transmission. The random backoff count is a pseudo-random integer,and may be set to one of 0 to CW values. In this case, CW refers to aContention Window parameter value. Although an initial value of the CWparameter is denoted by CWmin, the initial value may be doubled in caseof a transmission failure (for example, in the case in which ACK of thetransmission frame is not received). If the CW parameter value isdenoted by CWmax, CWmax is maintained until data transmission issuccessful, and at the same time it is possible to attempt to start datatransmission. If data transmission was successful, the CW parametervalue is reset to CWmin. Preferably, CW, CWmin, and CWmax are set to2n−1 (where n=0, 1, 2, . . . ).

If the random backoff process starts operation, the STA continuouslymonitors the medium while counting down the backoff slot in response tothe decided backoff count value. If the medium is monitored as theoccupied state, the countdown stops and waits for a predetermined time.If the medium is in the idle state, the remaining countdown restarts.

As shown in the example of FIG. 6, if a packet to be transmitted to MACof STA3 arrives at the STA3, the STA3 determines whether the medium isin the idle state during the DIFS, and may directly start frametransmission. In the meantime, the remaining STAs monitor whether themedium is in the busy state, and wait for a predetermined time. Duringthe predetermined time, data to be transmitted may occur in each ofSTA1, STA2, and STA5. If the medium is in the idle state, each STA waitsfor the DIFS time and then performs countdown of the backoff slot inresponse to a random backoff count value selected by each STA. Theexample of FIG. 6 shows that STA2 selects the lowest backoff count valueand STA1 selects the highest backoff count value. That is, after STA2finishes backoff counting, the residual backoff time of STA5 at a frametransmission start time is shorter than the residual backoff time ofSTA1. Each of STA1 and STA5 temporarily stops countdown while STA2occupies the medium, and waits for a predetermined time. If occupying ofthe STA2 is finished and the medium re-enters the idle state, each ofSTA1 and STA5 waits for a predetermined time DIFS, and restarts backoffcounting. That is, after the remaining backoff slot as long as theresidual backoff time is counted down, frame transmission may startoperation. Since the residual backoff time of STA5 is shorter than thatof STA1, STA5 starts frame transmission. Meanwhile, data to betransmitted may occur in STA4 while STA2 occupies the medium. In thiscase, if the medium is in the idle state, STA4 waits for the DIFS time,performs countdown in response to the random backoff count valueselected by the STA4, and then starts frame transmission. FIG. 6exemplarily shows the case in which the residual backoff time of STA5 isidentical to the random backoff count value of STA4 by chance. In thiscase, an unexpected collision may occur between STA4 and STA5. If thecollision occurs between STA4 and STA5, each of STA4 and STA5 does notreceive ACK, resulting in the occurrence of a failure in datatransmission. In this case, each of STA4 and STA5 increases the CW valuetwo times, and STA4 or STA5 may select a random backoff count value andthen perform countdown. Meanwhile, STA1 waits for a predetermined timewhile the medium is in the occupied state due to transmission of STA4and STA5. In this case, if the medium is in the idle state, STA1 waitsfor the DIFS time, and then starts frame transmission after lapse of theresidual backoff time.

STA Sensing Operation

As described above, the CSMA/CA mechanism includes not only a physicalcarrier sensing mechanism in which the AP and/or STA can directly sensethe medium, but also a virtual carrier sensing mechanism. The virtualcarrier sensing mechanism can solve some problems (such as a hidden nodeproblem) encountered in the medium access. For the virtual carriersensing, MAC of the WLAN system can utilize a Network Allocation Vector(NAV). In more detail, by means of the NAV value, the AP and/or STA,each of which currently uses the medium or has authority to use themedium, may inform another AP and/or another STA for the remaining timein which the medium is available. Accordingly, the NAV value maycorrespond to a reserved time in which the medium will be used by the APand/or STA configured to transmit the corresponding frame. An STA havingreceived the NAV value may prohibit or defer medium access (or channelaccess) during the corresponding reserved time. For example, NAV may beset according to the value of a ‘duration’ field of the MAC header ofthe frame.

The robust collision detect mechanism has been proposed to reduce theprobability of such collision, and as such a detailed descriptionthereof will hereinafter be described with reference to FIGS. 7 and 8.Although an actual carrier sensing range is different from atransmission range, it is assumed that the actual carrier sensing rangeis identical to the transmission range for convenience of descriptionand better understanding of the present invention.

FIG. 7 is a conceptual diagram illustrating a hidden node and an exposednode.

FIG. 7(a) exemplarily shows the hidden node. In FIG. 7(a), STA Acommunicates with STA B, and STA C has information to be transmitted. InFIG. 7(a), STA C may determine that the medium is in the idle state whenperforming carrier sensing before transmitting data to STA B, under thecondition that STA A transmits information to STA B. Since transmissionof STA A (i.e., occupied medium) may not be detected at the location ofSTA C, it is determined that the medium is in the idle state. In thiscase, STA B simultaneously receives information of STA A and informationof STA C, resulting in the occurrence of collision. Here, STA A may beconsidered as a hidden node of STA C.

FIG. 7(b) exemplarily shows an exposed node. In FIG. 7(b), under thecondition that STA B transmits data to STA A, STA C has information tobe transmitted to STA D. If STA C performs carrier sensing, it isdetermined that the medium is occupied due to transmission of STA B.Therefore, although STA C has information to be transmitted to STA D,the medium-occupied state is sensed, such that the STA C must wait for apredetermined time (i.e., standby mode) until the medium is in the idlestate. However, since STA A is actually located out of the transmissionrange of STA C, transmission from STA C may not collide withtransmission from STA B from the viewpoint of STA A, such that STA Cunnecessarily enters the standby mode until STA B stops transmission.Here, STA C is referred to as an exposed node of STA B.

FIG. 8 is a conceptual diagram illustrating RTS (Request To Send) andCTS (Clear To Send).

In order to efficiently utilize the collision avoidance mechanism underthe above-mentioned situation of FIG. 7, it is possible to use a shortsignaling packet such as RTS (request to send) and CTS (clear to send).RTS/CTS between two STAs may be overheard by peripheral STA(s), suchthat the peripheral STA(s) may consider whether information iscommunicated between the two STAs. For example, if STA to be used fordata transmission transmits the RTS frame to the STA having receiveddata, the STA having received data transmits the CTS frame to peripheralSTAs, and may inform the peripheral STAs that the STA is going toreceive data.

FIG. 8(a) exemplarily shows the method for solving problems of thehidden node. In FIG. 8(a), it is assumed that each of STA A and STA C isready to transmit data to STA B. If STA A transmits RTS to STA B, STA Btransmits CTS to each of STA A and STA C located in the vicinity of theSTA B. As a result, STA C must wait for a predetermined time until STA Aand STA B stop data transmission, such that collision is prevented fromoccurring.

FIG. 8(b) exemplarily shows the method for solving problems of theexposed node. STA C performs overhearing of RTS/CTS transmission betweenSTA A and STA B, such that STA C may determine no collision although ittransmits data to another STA (for example, STA D). That is, STA Btransmits an RTS to all peripheral STAs, and only STA A having data tobe actually transmitted can transmit a CTS. STA C receives only the RTSand does not receive the CTS of STA A, such that it can be recognizedthat STA A is located outside of the carrier sensing range of STA C.

Power Management

As described above, the WLAN system has to perform channel sensingbefore STA performs data transmission/reception. The operation of alwayssensing the channel causes persistent power consumption of the STA.There is not much difference in power consumption between the reception(Rx) state and the transmission (Tx) state. Continuous maintenance ofthe Rx state may cause large load to a power-limited STA (i.e., STAoperated by a battery). Therefore, if STA maintains the Rx standby modeso as to persistently sense the channel, power is inefficiently consumedwithout special advantages in terms of WLAN throughput. In order tosolve the above-mentioned problem, the WLAN system supports a powermanagement (PM) mode of the STA.

The PM mode of the STA is classified into an active mode and a PowerSave (PS) mode. The STA is basically operated in the active mode. TheSTA operating in the active mode maintains an awake state. If the STA isin the awake state, the STA may normally operate such that it canperform frame transmission/reception, channel scanning, or the like. Onthe other hand, STA operating in the PS mode is configured to switchfrom the doze state to the awake state or vice versa. STA operating inthe sleep state is operated with minimum power, and the STA does notperform frame transmission/reception and channel scanning.

The amount of power consumption is reduced in proportion to a specifictime in which the STA stays in the sleep state, such that the STAoperation time is increased in response to the reduced powerconsumption. However, it is impossible to transmit or receive the framein the sleep state, such that the STA cannot mandatorily operate for along period of time. If there is a frame to be transmitted to the AP,the STA operating in the sleep state is switched to the awake state,such that it can transmit/receive the frame in the awake state. On theother hand, if the AP has a frame to be transmitted to the STA, thesleep-state STA is unable to receive the frame and cannot recognize thepresence of a frame to be received. Accordingly, STA may need to switchto the awake state according to a specific period in order to recognizethe presence or absence of a frame to be transmitted to the STA (or inorder to receive a signal indicating the presence of the frame on theassumption that the presence of the frame to be transmitted to the STAis decided).

FIG. 9 is a conceptual diagram illustrating a power management (PM)operation.

Referring to FIG. 9, AP 210 transmits a beacon frame to STAs present inthe BSS at intervals of a predetermined time period in steps (S211,S212, S213, S214, S215, S216). The beacon frame includes a TIMinformation element. The TIM information element includes bufferedtraffic regarding STAs associated with the AP 210, and includes specificinformation indicating that a frame is to be transmitted. The TIMinformation element includes a TIM for indicating a unicast frame and aDelivery Traffic Indication Map (DTIM) for indicating a multicast orbroadcast frame.

AP 210 may transmit a DTIM once whenever the beacon frame is transmittedthree times. Each of STA1 220 and STA2 222 is operated in the PS mode.Each of STA1 220 and STA2 222 is switched from the sleep state to theawake state every wakeup interval, such that STA1 220 and STA2 222 maybe configured to receive the TIM information element transmitted by theAP 210. Each STA may calculate a switching start time at which each STAmay start switching to the awake state on the basis of its own localclock. In FIG. 9, it is assumed that a clock of the STA is identical toa clock of the AP.

For example, the predetermined wakeup interval may be configured in sucha manner that STA1 220 can switch to the awake state to receive the TIMelement every beacon interval. Accordingly, STA1 220 may switch to theawake state in step S221 when AP 210 first transmits the beacon frame instep S211. STA1 220 receives the beacon frame, and obtains the TIMinformation element. If the obtained TIM element indicates the presenceof a frame to be transmitted to STA1 220, STA1 220 may transmit a PowerSave-Poll (PS-Poll) frame, which requests the AP 210 to transmit theframe, to the AP 210 in step S221 a. The AP 210 may transmit the frameto STA 1 220 in response to the PS-Poll frame in step S231. STA1 220having received the frame is re-switched to the sleep state, andoperates in the sleep state.

When AP 210 secondly transmits the beacon frame, a busy medium state inwhich the medium is accessed by another device is obtained, the AP 210may not transmit the beacon frame at an accurate beacon interval and maytransmit the beacon frame at a delayed time in step S212. In this case,although STA1 220 is switched to the awake state in response to thebeacon interval, it does not receive the delay-transmitted beacon frameso that it re-enters the sleep state in step S222.

When AP 210 thirdly transmits the beacon frame, the corresponding beaconframe may include a TIM element denoted by DTIM. However, since the busymedium state is given, AP 210 transmits the beacon frame at a delayedtime in step S213. STA1 220 is switched to the awake state in responseto the beacon interval, and may obtain a DTIM through the beacon frametransmitted by the AP 210. It is assumed that DTIM obtained by STA1 220does not have a frame to be transmitted to STA1 220 and there is a framefor another STA. In this case, STA1 220 confirms the absence of a frameto be received in the STA1 220, and re-enters the sleep state, such thatthe STA1 220 may operate in the sleep state. After the AP 210 transmitsthe beacon frame, the AP 210 transmits the frame to the correspondingSTA in step S232.

AP 210 fourthly transmits the beacon frame in step S214. However, it isimpossible for STA1 220 to obtain information regarding the presence ofbuffered traffic associated with the STA1 220 through double receptionof a TIM element, such that the STA1 220 may adjust the wakeup intervalfor receiving the TIM element. Alternatively, provided that signalinginformation for coordination of the wakeup interval value of STA1 220 iscontained in the beacon frame transmitted by AP 210, the wakeup intervalvalue of the STA1 220 may be adjusted. In this example, STA1 220, thathas been switched to receive a TIM element every beacon interval, may beswitched to another operation state in which STA1 220 can awake from thesleep state once every three beacon intervals. Therefore, when AP 210transmits a fourth beacon frame in step S214 and transmits a fifthbeacon frame in step S215, STA1 220 maintains the sleep state such thatit cannot obtain the corresponding TIM element.

When AP 210 sixthly transmits the beacon frame in step S216, STA1 220 isswitched to the awake state and operates in the awake state, such thatthe STA1 220 is unable to obtain the TIM element contained in the beaconframe in step S224. The TIM element is a DTIM indicating the presence ofa broadcast frame, such that STA1 220 does not transmit the PS-Pollframe to the AP 210 and may receive a broadcast frame transmitted by theAP 210 in step S234. In the meantime, the wakeup interval of STA2 230may be longer than a wakeup interval of STA1 220. Accordingly, STA2 230enters the awake state at a specific time S215 where the AP 210 fifthlytransmits the beacon frame, such that the STA2 230 may receive the TIMelement in step S241. STA2 230 recognizes the presence of a frame to betransmitted to the STA2 230 through the TIM element, and transmits thePS-Poll frame to the AP 210 so as to request frame transmission in stepS241 a. AP 210 may transmit the frame to STA2 230 in response to thePS-Poll frame in step S233.

In order to operate/manage the power save (PS) mode shown in FIG. 9, theTIM element may include either a TIM indicating the presence or absenceof a frame to be transmitted to the STA, or a DTIM indicating thepresence or absence of a broadcast/multicast frame. DTIM may beimplemented through field setting of the TIM element.

FIGS. 10 to 12 are conceptual diagrams illustrating detailed operationsof the STA having received a Traffic Indication Map (TIM).

Referring to FIG. 10, STA is switched from the sleep state to the awakestate so as to receive the beacon frame including a TIM from the AP. STAinterprets the received TIM element such that it can recognize thepresence or absence of buffered traffic to be transmitted to the STA.After STA contends with other STAs to access the medium for PS-Pollframe transmission, the STA may transmit the PS-Poll frame forrequesting data frame transmission to the AP. The AP having received thePS-Poll frame transmitted by the STA may transmit the frame to the STA.STA may receive a data frame and then transmit an ACK frame to the AP inresponse to the received data frame. Thereafter, the STA may re-enterthe sleep state.

As can be seen from FIG. 10, the AP may operate according to theimmediate response scheme, such that the AP receives the PS-Poll framefrom the STA and transmits the data frame after lapse of a predeterminedtime [for example, Short Inter-Frame Space (SIFS)]. In contrast, the APhaving received the PS-Poll frame does not prepare a data frame to betransmitted to the STA during the SIFS time, such that the AP mayoperate according to the deferred response scheme, and as such adetailed description thereof will hereinafter be described withreference to FIG. 11.

The STA operations of FIG. 11 in which the STA is switched from thesleep state to the awake state, receives a TIM from the AP, andtransmits the PS-Poll frame to the AP through contention are identicalto those of FIG. 10. If the AP having received the PS-Poll frame doesnot prepare a data frame during the SIFS time, the AP may transmit theACK frame to the STA instead of transmitting the data frame. If the dataframe is prepared after transmission of the ACK frame, the AP maytransmit the data frame to the STA after completion of such contending.STA may transmit the ACK frame indicating successful reception of a dataframe to the AP, and may be shifted to the sleep state.

FIG. 12 shows the exemplary case in which AP transmits DTIM. STAs may beswitched from the sleep state to the awake state so as to receive thebeacon frame including a DTIM element from the AP. STAs may recognizethat multicast/broadcast frame(s) will be transmitted through thereceived DTIM. After transmission of the beacon frame including theDTIM, AP may directly transmit data (i.e., multicast/broadcast frame)without transmitting/receiving the PS-Poll frame. While STAscontinuously maintains the awake state after reception of the beaconframe including the DTIM, the STAs may receive data, and then switch tothe sleep state after completion of data reception.

TIM Structure

In the operation and management method of the Power save (PS) mode basedon the TIM (or DTIM) protocol shown in FIGS. 9 to 12, STAs may determinethe presence or absence of a data frame to be transmitted for the STAsthrough STA identification information contained in the TIM element. STAidentification information may be specific information associated withan Association Identifier (AID) to be allocated when an STA isassociated with an AP.

AID is used as a unique ID of each STA within one BSS. For example, AIDfor use in the current WLAN system may be allocated to one of 1 to 2007.In the case of the current WLAN system, 14 bits for AID may be allocatedto a frame transmitted by AP and/or STA. Although the AID value may beassigned a maximum of 16383, the values of 2008˜16383 are set toreserved values.

The TIM element according to legacy definition is inappropriate forapplication of M2M application through which many STAs (for example, atleast 2007 STAs) are associated with one AP. If the conventional TIMstructure is extended without any change, the TIM bitmap sizeexcessively increases, such that it is impossible to support theextended TIM structure using the legacy frame format, and the extendedTIM structure is inappropriate for M2M communication in whichapplication of a low transfer rate is considered. In addition, it isexpected that there are a very small number of STAs each having an Rxdata frame during one beacon period. Therefore, according to exemplaryapplication of the above-mentioned M2M communication, it is expectedthat the TIM bitmap size is increased and most bits are set to zero (0),such that there is needed a technology capable of efficientlycompressing such bitmap.

In the legacy bitmap compression technology, successive values (each ofwhich is set to zero) of 0 are omitted from a head part of bitmap, andthe omitted result may be defined as an offset (or start point) value.However, although STAs each including the buffered frame is small innumber, if there is a high difference between AID values of respectiveSTAs, compression efficiency is not high. For example, assuming that theframe to be transmitted to only a first STA having an AID of 10 and asecond STA having an AID of 2000 is buffered, the length of a compressedbitmap is set to 1990, the remaining parts other than both edge partsare assigned zero (0). If STAs associated with one AP is small innumber, inefficiency of bitmap compression does not cause seriousproblems. However, if the number of STAs associated with one APincreases, such inefficiency may deteriorate overall system throughput.

In order to solve the above-mentioned problems, AIDs are divided into aplurality of groups such that data can be more efficiently transmittedusing the AIDs. A designated group ID (GID) is allocated to each group.AIDs allocated on the basis of such group will hereinafter be describedwith reference to FIG. 13.

FIG. 13(a) is a conceptual diagram illustrating a group-based AID. InFIG. 13(a), some bits located at the front part of the AID bitmap may beused to indicate a group ID (GID). For example, it is possible todesignate four GIDs using the first two bits of an AID bitmap. If atotal length of the AID bitmap is denoted by N bits, the first two bits(B1 and B2) may represent a GID of the corresponding AID.

FIG. 13(b) is a conceptual diagram illustrating a group-based AID. InFIG. 13(b), a GID may be allocated according to the position of AID. Inthis case, AIDs having the same GID may be represented by offset andlength values. For example, if GID 1 is denoted by Offset A and LengthB, this means that AIDs (A˜A+B−1) on bitmap are respectively set to GID1. For example, FIG. 13(b) assumes that AIDs (1˜N4) are divided intofour groups. In this case, AIDs contained in GID 1 are denoted by 1˜N1,and the AIDs contained in this group may be represented by Offset 1 andLength N1. AIDs contained in GID 2 may be represented by Offset (N1+1)and Length (N2−N1+1), AIDs contained in GID 3 may be represented byOffset (N2+1) and Length (N3−N2+1), and AIDs contained in GID 4 may berepresented by Offset (N3+1) and Length (N4−N3+1).

In case of using the aforementioned group-based AIDs, channel access isallowed in a different time interval according to individual GIDs, theproblem caused by the insufficient number of TIM elements compared witha large number of STAs can be solved and at the same time data can beefficiently transmitted/received. For example, during a specific timeinterval, channel access is allowed only for STA(s) corresponding to aspecific group, and channel access to the remaining STA(s) may berestricted. A predetermined time interval in which access to onlyspecific STA(s) is allowed may also be referred to as a RestrictedAccess Window (RAW).

Channel access based on GID will hereinafter be described with referenceto FIG. 13(c). If AIDs are divided into three groups, the channel accessmechanism according to the beacon interval is exemplarily shown in FIG.13(c). A first beacon interval (or a first RAW) is a specific intervalin which channel access to an STA corresponding to an AID contained inGID 1 is allowed, and channel access of STAs contained in other GIDs isdisallowed. For implementation of the above-mentioned structure, a TIMelement used only for AIDs corresponding to GID 1 is contained in afirst beacon frame. A TIM element used only for AIDs corresponding toGID 2 is contained in a second beacon frame. Accordingly, only channelaccess to an STA corresponding to the AID contained in GID 2 is allowedduring a second beacon interval (or a second RAW) during a second beaconinterval (or a second RAW). A TIM element used only for AIDs having GID3 is contained in a third beacon frame, such that channel access to anSTA corresponding to the AID contained in GID 3 is allowed using a thirdbeacon interval (or a third RAW). A TIM element used only for AIDs eachhaving GID 1 is contained in a fourth beacon frame, such that channelaccess to an STA corresponding to the AID contained in GID 1 is allowedusing a fourth beacon interval (or a fourth RAW). Thereafter, onlychannel access to an STA corresponding to a specific group indicated bythe TIM contained in the corresponding beacon frame may be allowed ineach of beacon intervals subsequent to the fifth beacon interval (or ineach of RAWs subsequent to the fifth RAW).

Although FIG. 13(c) exemplarily shows that the order of allowed GIDs isperiodical or cyclical according to the beacon interval, the scope orspirit of the present invention is not limited thereto. That is, onlyAID(s) contained in specific GID(s) may be contained in a TIM element,such that channel access to STA(s) corresponding to the specific AID(s)is allowed during a specific time interval (for example, a specificRAW), and channel access to the remaining STA(s) is disallowed.

The aforementioned group-based AID allocation scheme may also bereferred to as a hierarchical structure of a TIM. That is, a total AIDspace is divided into a plurality of blocks, and channel access toSTA(s) (i.e., STA(s) of a specific group) corresponding to a specificblock having any one of the remaining values other than ‘0’ may beallowed. Therefore, a large-sized TIM is divided into small-sizedblocks/groups, STA can easily maintain TIM information, andblocks/groups may be easily managed according to class, QoS or usage ofthe STA. Although FIG. 13 exemplarily shows a 2-level layer, ahierarchical TIM structure comprised of two or more levels may beconfigured. For example, a total AID space may be divided into aplurality of page groups, each page group may be divided into aplurality of blocks, and each block may be divided into a plurality ofsub-blocks. In this case, according to the extended version of FIG.13(a), first N1 bits of AID bitmap may represent a page ID (i.e., PID),the next N2 bits may represent a block ID, the next N3 bits mayrepresent a sub-block ID, and the remaining bits may represent theposition of STA bits contained in a sub-block.

In the examples of the present invention, various schemes for dividingSTAs (or AIDs allocated to respective STAs) into predeterminedhierarchical group units, and managing the divided result may be appliedto the embodiments, however, the group-based AID allocation scheme isnot limited to the above examples.

PPDU Frame Format

A Physical Layer Convergence Protocol (PLCP) Packet Data Unit (PPDU)frame format may include a Short Training Field (STF), a Long TrainingField (LTF), a signal (SIG) field, and a data field. The most basic (forexample, non-HT) PPDU frame format may be comprised of a Legacy-STF(L-STF) field, a Legacy-LTF (L-LTF) field, an SIG field, and a datafield. In addition, the most basic PPDU frame format may further includeadditional fields (i.e., STF, LTF, and SIG fields) between the SIG fieldand the data field according to the PPDU frame format types (forexample, HT-mixed format PPDU, HT-greenfield format PPDU, a VHT PPDU,and the like).

STF is a signal for signal detection, Automatic Gain Control (AGC),diversity selection, precise time synchronization, etc. LTF is a signalfor channel estimation, frequency error estimation, etc. The sum of STFand LTF may be referred to as a PCLP preamble. The PLCP preamble may bereferred to as a signal for synchronization and channel estimation of anOFDM physical layer.

The SIG field may include a RATE field, a LENGTH field, etc. The RATEfield may include information regarding data modulation and coding rate.The LENGTH field may include information regarding the length of data.Furthermore, the SIG field may include a parity field, a SIG TAIL bit,etc.

The data field may include a service field, a PLCP Service Data Unit(PSDU), and a PPDU TAIL bit. If necessary, the data field may furtherinclude a padding bit. Some bits of the SERVICE field may be used tosynchronize a descrambler of the receiver. PSDU may correspond to a MACPDU defined in the MAC layer, and may include data generated/used in ahigher layer. A PPDU TAIL bit may allow the encoder to return to a stateof zero (0). The padding bit may be used to adjust the length of a datafield according to a predetermined unit.

MAC PDU may be defined according to various MAC frame formats, and thebasic MAC frame is composed of a MAC header, a frame body, and a FrameCheck Sequence. The MAC frame is composed of MAC PDUs, such that it canbe transmitted/received through PSDU of a data part of the PPDU frameformat.

On the other hand, a null-data packet (NDP) frame format may indicate aframe format having no data packet. That is, the NDP frame includes aPLCP header part (i.e., STF, LTF, and SIG fields) of a general PPDUformat, whereas it does not include the remaining parts (i.e., the datafield). The NDP frame may be referred to as a short frame format.

Short Beacon

A normal beacon frame is composed of a MAC header, a frame body and anFCS and the frame body may include the following fields.

A timestamp field is for synchronization and all STAs that have receiveda beacon frame can change/update local clock signals thereof accordingto a timestamp value.

A beacon interval field indicates an interval between beacontransmissions and is represented as a time unit (TU). The TU may berepresented in microseconds and can be defined as 1024 μs, for example.A time when an AP needs to transmit a beacon can be represented as atarget beacon transmission time (TBTT). That is, a beacon interval fieldcorresponds to an interval between a beacon frame transmission time andthe next TBTT. An STA that has received a previous beacon can calculatea transmission time of the next beacon from the beacon interval field.In general, a beacon interval can be set to 100 TU.

Capability information field includes information about capabilities ofa device/network. For example, the network type of an ad hoc orinfrastructure network can be indicated through the capabilityinformation field. Further, the capability information field may be usedto signal whether polling is supported, details of encryption and thelike.

In addition, the beacon frame can include an SSID, supported rates,frequency hopping (FH) parameter set, direct sequence spread spectrum(DSSS) parameter set, contention free (CF) parameter set, IBSS parameterset, TIM, country IE, power constraint, QoS capability, high-throughput(HT) capability and the like. However, the aforementionedfields/information included in the beacon frame are exemplary and thebeacon frame mentioned in the present invention is not limited thereto.

Distinguished from the above-described normal beacon frame, a shortbeacon frame can be defined. A conventional normal beacon may bereferred to as a full beacon to be discriminated from the short beacon.

FIG. 14 is a diagram illustrating the short beacon.

A short beacon interval is represented in TUs and a beacon interval(i.e. beacon interval of the full beacon) can be defined as an integermultiple of the short beacon interval. As shown in FIG. 14, the fullbeacon interval can be defined as Full Beacon Interval=N*Short BeaconInterval (here, N≧1). For example, the short beacon can be transmittedmore than once during an interval between successive full beacontransmission times. FIG. 14 illustrates an example in which the shortbeacon Short B is transmitted three times during the full beaconinterval.

The STA may determine whether a desired network is available using anSSID (or compressed SSID) included in a short beacon. The STA maytransmit an association request to a MAC address of an AP, which isincluded in the short beacon transmitted from the desired network. Sincethe short beacon is transmitted more frequently than the full beacon, ingeneral, an unassociated STA can rapidly associate with a desired AP bysupporting the short beacon. When the STA needs additional informationfor association, the STA can transmit a probe request to a desired AP.Further, the STA can perform synchronization using timestamp informationincluded in the short beacon. In addition, whether system information(or network information or system parameters (system/network information(parameters) are collectively referred to as “system information”hereinafter)) has been changed can be signaled through the short beacon.When the system information has been changed, the STA may acquire thechanged system information through a full beacon. The short beacon mayinclude a TIM. That is, the TIM may be provided through the full beaconor the short beacon.

FIG. 15 is a diagram illustrating exemplary fields included in the shortbeacon frame.

A frame control (FC) field may include protocol version, type, subtype,next full beacon present, SSID present, BSS bandwidth (BW) and securityfields. The FC may have a length of 2 octets

From among the subfields of the FC field, the protocol version field hasa length of 2 bits and may be set to 0 by default. The type field andthe subtype field are respectively defined as 2-bit and 4-bit fields andcan indicate the function of the corresponding frame together (forexample, the type field and the subtype field can indicate that thecorresponding frame is a short beacon frame). The next full beaconpresent field is defined as a 1-bit field and can be set to a valueindicating whether a duration to next full beacon field (or informationabout the next TBTT) is included in the short beacon frame. The SSIDpresent field is defined as a 1-bit field and can be set to a valueindicating whether a compressed SSID field is present in the shortbeacon frame. The BSS BW field is defined as a 3-bit field and can beset to a value indicating a current operation bandwidth (e.g. 1, 2, 4, 8or 16 MHz) of a BSS. The security field is defined as a 1-bit field andcan be set to a value indicating whether the corresponding AP is an RSNAAP. The remaining bits (e.g. 2 bits) may be reserved.

A sound address (SA) field in the short beacon frame may be a MACaddress of an AP that transmits the short beacon. The SA may have alength of 6 octets.

A timestamp field in the short beacon frame may include LSB (LeastSignificant Bit) 4 bytes (i.e. 4 octets) of the timestamp of the AP.Even when only the LSB 4 bytes are provided, instead of all timestampvalues, an STA that has received all timestamp values (e.g. associatedSTA) can perform synchronization using the LSB 4 bytes.

A change sequence field in the short beacon frame may includeinformation for signaling whether system information has been changed.Specifically, when critical information (e.g. full beacon information)of the network is changed, a change sequence counter increases by 1.This field is defined as a 1-octet field.

A duration to next full beacon field may be included in the short beaconor not. This field can signal, to the STA, a duration from atransmission time of the corresponding short beacon to a transmissiontime of the next full beacon. Accordingly, the STA that has listened tothe short beacon may reduce power consumption by operating in a doze (orsleep) mode until the next full beacon. Alternatively, the duration tonext full beacon field may be configured as information indicating thenext TBTT. The length of this field can be defined as 3 octets, forexample

A compressed SSID field may be included in the short beacon or not. Thisfield may include part or a hash of the SSID of the correspondingnetwork. An STA that already knew the corresponding network can beallowed to discover the network using the SSID. The length of this fieldcan be defined as 4 octets, for example.

The short beacon frame may include additional or optional fields orinformation elements (IEs) in addition to the aforementioned exemplaryfields.

A forward error correction (FFC) field included in the short beaconframe can be used to check whether the short beacon frame has an errorand may be configured as an FCS field. This field can be defined as a4-octet field.

Improved System Information Update Method

While an AP periodically transmits a full beacon frame including systeminformation in a conventional wireless LAN environment, the full beaconframe including the system information may not be periodicallytransmitted all the time in an enhanced wireless LAN environment. Forexample, a beacon may not be transmitted when an associated STA is notpresent in a home LAN environment. Even if the full beacon frame isperiodically transmitted, the short beacon may not include the durationto next full beacon field in order to reduce overhead of the shortbeacon. In this case, the AP can set the next full beacon present fieldin the FC field of the short beacon frame to 0 and transmit the shortbeacon that does not include the duration to next full beacon field.

In this case, when the AP does not notify the STA that the full beaconis not transmitted, the STA repeats attempting and failing to receivethe full beacon and thus power consumption of the STA may increase.Further, when the short beacon does not include information about a timewhen the next full beacon can be received, the STA continuously attemptsto receive the full beacon until the full beacon is actually transmittedeven though the STA has received the short beacon. This may increasepower consumption of the STA. Accordingly, power consumption of the STAcan be reduced when the AP rapidly informs the STA that the AP does nottransmit the full beacon or transmission of the next full beacon is notperiodically performed.

In addition, when the STA determines that the AP does not transmit thefull beacon, the STA can obtain system information through a proberequest/response operation, instead of waiting for the full beacon, andefficiently perform association with the corresponding AP. For example,upon reception of a probe request frame from the STA, the AP cantransmit a probe response frame including system information (e.g. SSID,supported rates, FH parameter set, DSSS parameter set, CF parameter set,IBSS parameter set, country IE and the like) to the STA in response tothe probe request frame. Accordingly, the STA can obtain the systeminformation provided through the probe response frame and associate withthe corresponding AP by performing association request/response.

Since the full beacon including the system information is periodicallytransmitted in conventional wireless LAN operation, the STA can obtainchanged system information by receiving the next beacon when the systeminformation is changed. In an environment in which the full beaconincluding the system information is not periodically transmitted,however, the STA may not obtain the changed system information at anappropriate time. In this case, the STA cannot correctly operate in thecorresponding wireless LAN network.

The present invention provides a method by which an STA can correctlyobtain changed system information and retain updated system informationin a system in which an AP does not periodically transmit a full beaconframe (i.e. a frame including the system information).

Embodiment 1

The present embodiment relates to a method by which the AP notifies theSTA whether the full beacon frame including the system information isperiodically transmitted.

For example, information indicating whether the full beacon frame isperiodically transmitted can be included in a short beacon frame andsignaled to the STA.

FIG. 16 illustrates a short beacon frame format according to anembodiment of the present invention.

As shown in FIG. 16, a full beacon present field may be defined in an FCfield of a short beacon frame. The full beacon present field may be setto a value indicating whether a periodically transmitted full beacon ispresent. For example, when the AP transmits a full beacon (orperiodically transmits the full beacon), the value of the full beaconpresent field can be set to 1. When the value of the full beacon presentfield is set to 0, this value can mean that the AP does not transmit thefull beacon (or does not periodically transmit the full beacon). Whenthe value of the full beacon present field is set to 0, a next fullbeacon present field in the FC field of the short beacon frame can beset to a value (e.g. 0) indicating that a duration to next full beaconfield is not present in the short beacon frame.

FIG. 17 illustrates a short beacon frame format according to anotherembodiment of the present invention.

As shown in FIG. 17, when the next full present field in the FC field ofthe short beacon is set to 1 and the duration to next full beacon fieldhas a predetermined value (for example, all bits are set to 0 or 1),this can indicate that a full beacon is not transmitted (or the fullbeacon is not periodically transmitted). Distinguished from the exampleof FIG. 16 in which an explicit field indicating presence or absence ofthe full beacon is additionally defined, the example of FIG. 17 may beconsidered to be a method of implicitly indicating absence of the fullbeacon when values of existing fields constitute a specific combination.

The example of FIG. 17 shows that the AP does not transmit the fullbeacon when the value of the duration to next full beacon field is setto 0. In this case, the duration to next full beacon field needs to beincluded in the short beacon frame all the time even though the AP doesnot transmit the full beacon.

Embodiment 2

The present embodiment describes operations of an AP and an STAaccording to whether a full beacon is transmitted.

FIG. 18 is a diagram illustrating a method for transmitting andreceiving a full beacon frame according to an embodiment of the presentinvention.

The AP may not transmit a full beacon when an STA associated with the APis not present. In this case, the AP may inform the STA that the fullbeacon is not transmitted through a specific field of a short beacon(for example, by setting the value of the duration to next full beaconfield to 0, as shown in FIG. 17).

When an STA associated with the AP is present, the AP starts to transmitthe full beacon. In this case, the duration to next full beacon field ofthe short beacon frame can be set to a value (e.g. a non-zero value)that indicates a transmission time of the next full beacon and the STAthat has received the short beacon can determine a reception time of thenext full beacon.

When the AP does not transmit the full beacon, as shown in the exampleof FIG. 16, the duration to next full beacon field may not be includedin the short beacon frame and the next full beacon present field may beset to 0. Upon reception of this short beacon frame, the STA maydetermine that the full beacon is not transmitted and thus the STA canimmediately perform active scanning without waiting for the full beacon.Alternatively, upon determining that the full beacon is not transmittedfrom information included in the short beacon frame, the STA may performactive scanning when the STA waits for a predetermined time (e.g. 100 ms(i.e. a default beacon interval)) from when the short beacon is receivedand then does not receive the full beacon for the predetermined time.

The STA may transmit a probe request frame to the AP through activescanning, receive a probe response frame from the AP as a response tothe probe request frame and acquire system information included in theprobe response frame. The probe response frame may include information(e.g. change sequence (or version) information) indicating whether thesystem information is changed. The change sequence information may becalled (AP) configuration change count (CCC) since the change sequenceinformation is incremented by 1 whenever the system information ischanged.

FIG. 19 is a diagram illustrating a method for transmitting andreceiving a full beacon frame according to another embodiment of thepresent invention.

When the STA determines that the AP does not transmit the full beaconframe from information included in the short beacon frame (e.g. as shownin the example of FIG. 16 or FIG. 17), the STA may request the AP totransmit the full beacon frame.

To this end, the STA may transmit a full beacon request frame to the AP.Upon reception of the full beacon request frame, the AP may start totransmit the full beacon frame in response to the full beacon requestframe.

For example, the AP can periodically transmit the full beacon frame fora predetermined time or a predetermined number of times after receptionof the full beacon request frame from the STA. The predeterminedtime/predetermined number of times may be set according to a valuerequested by the STA or on the basis of a value predetermined accordingto system characteristics.

FIG. 20 is a diagram illustrating a method for transmitting andreceiving a full beacon frame according to another embodiment of thepresent invention.

As described above with reference to FIG. 18, when the AP cannotimmediately start to transmit the full beacon frame upon reception ofthe full beacon request frame from the STA, the AP can transmit the fullbeacon response frame to the STA. The full beacon response frame mayinclude information (e.g. the duration to next full beacon field or nextTBTT field) used for the STA to determine a transmission time of thenext full beacon. Accordingly, the STA can determine a reception time ofthe next full beacon.

While the STA transmits the full beacon request frame to the AP torequest the AP to transmit the full beacon in the examples of FIGS. 19and 20, the STA may request the AP to transmit the full beacon throughthe probe request frame. That is, the STA can request the AP to transmitthe full beacon by transmitting the probe request frame to the AP upondetermining that the AP does not transmit the full beacon. To this end,the probe request frame may include information indicating that the STArequests transmission of the full beacon frame. Upon reception of theprobe request frame including the information, the AP may transmit thefull beacon frame to the STA and, when the AP cannot immediatelytransmit the full beacon frame, may provide information used for the STAto determine a time when the next full beacon can be received by the STAby transmitting the probe response frame.

That is, the STA can transmit the full beacon request frame/proberequest frame to the AP in order to request the full beacon frame upondetermining that the AP does not transmit the full beacon frame. Inresponse to the full beacon request frame/probe request frame, the APcan transmit the full beacon frame/full beacon response frame/proberesponse frame.

Here, the AP can transmit the full beacon response frame/probe responseframe to the STA in a unicast or broadcast manner.

FIG. 21 is a diagram illustrating broadcast transmission of the proberesponse frame.

In conventional wireless LAN systems, the probe response frame istransmitted as a response to the probe request frame in a unicast manneronly for an STA that has transmitted the probe request frame. However,since the probe response frame may provide information on a transmissiontime of the next full beacon, like the full beacon response frame, asproposed in the present invention, it may be appropriate to broadcastthe probe response frame.

Information that indicates/requests transmission of the probe responseframe in a broadcast manner (information indicating broadcast of proberesponse in the example of FIG. 21) may be included in the probe requestframe. In this case, the AP can transmit the probe response frame in abroadcast manner.

For example, the value of a reception address field of the proberesponse frame can be set to a broadcast identifier (e.g. a wildcard).In addition, a most robust modulation and coding method (e.g. quadraturephase shift keying (QPSK) 1/12, 2 repetition) can be applied to data ofthe probe response frame transmitted in a broadcast manner such that allSTAs in a BSS can receive the data.

Embodiment 3

Since the full beacon including the system information is periodicallytransmitted in conventional wireless LAN operation, an STA can obtainchanged system information by receiving the next full beacon when thesystem information has been changed. However, when the full beaconincluding the system information is not periodically transmitted or thefull beacon is not transmitted and only the short beacon is transmitted,the STA cannot immediately update the system information even when thesystem information has been changed.

The present invention proposes a method for updating changed systeminformation by the STA when the system information has been changed in asystem in which the full beacon is not transmitted.

In a wireless LAN system (e.g. IEEE 802.11ah system) using the shortbeacon frame, the full beacon frame may be defined such that the fullbeacon frame includes information indicating whether the systeminformation is changed.

The information indicating whether the system information is changed maybe defined as a change sequence field or configuration change sequencefield, as shown in FIG. 22. The change sequence field may be set to avalue indicating whether the system information is changed.Specifically, when system information (e.g. non-dynamic systeminformation) other than dynamic elements (dynamic system information)such as timestamp information is changed, the change sequence field isdefined such that the value thereof increments by 1 and may have a valuein the range of 0 to 255 (that is, modulo 256 is applied). As describedabove, the change sequence field may be call the (AP) configurationchange count (CCC) field since the change sequence field is counted by 1whenever the system information is changed.

When the change sequence value included in the beacon frame or proberesponse frame is maintained as the same as a previous value, the STAcan immediately determine that the remaining fields included in thebeacon frame or probe response frame have not been changed and maydisregard the remaining fields. However, the STA can operate to obtaindynamic information such as a timestamp value even when the changesequence value has not been changed.

According to the present invention, the probe response frame may bedefined such that information (e.g. change sequence field) indicatingwhether the system information has been changed is included therein.That is, when the AP transmits the probe response frame as a response tothe probe request frame transmitted from the STA, the AP can include achange sequence, which corresponds to the system information included inthe probe response frame, in the probe response frame and transmit theprobe response frame.

Accordingly, when the STA acquires the system information through thefull beacon frame or the probe response frame, the STA can store thechange sequence value associated with the acquired system informationalong with the system information. Thereafter, when the STA receives theshort beacon frame or the full beacon frame, the STA can compare thechange sequence value stored therein with a change sequence valueincluded in the short beacon frame or the full beacon frame. When thetwo values are identical to each other, the STA can determine that thesystem information has not been changed. When the two values differ fromeach other, the STA can update the changed system information.

Here, when the full beacon frame is transmitted, the STA can obtain thechanged system information through the full beacon frame. However, theSTA cannot obtain the changed system information through the full beaconframe when the full beacon frame is not transmitted. Accordingly, thefollowing procedure may be performed in order to update the changedsystem information when the full beacon frame is not transmitted.

Embodiment 3-1

The present embodiment relates to a method for updating systeminformation using a probe request/response procedure.

The conventional probe request/response procedure is performed foractive scanning when an STA discovers an AP. The present inventionproposes use of the probe request/response procedure for systeminformation update. That is, while the conventional proberequest/response procedure is performed in order to associate an STAthat is not associated with an AP with the AP, an STA associated withthe AP can transmit a probe request to the AP and receive a proberesponse from the AP for system information update according to thepresent invention.

FIG. 23 is a diagram illustrating a probe request/response procedureaccording to an embodiment of the present invention.

An STA associated with the AP may receive a short beacon and thenconfirm whether system information has been changed by checking a changesequence value. When a change sequence value stored in the STA is 1whereas the change sequence value included in the short beacon is 2, asshown in FIG. 23, the STA can determine that the system information hasbeen changed.

In this case, the STA can transmit the probe request frame to the AP.Here, the probe request frame may further include information indicatingthat the probe request frame is a probe request frame for updating thesystem information.

The AP can transmit the probe response frame to the STA in response tothe probe request frame from the STA. Here, the AP can include currentsystem information (i.e. updated/changed system information) in theprobe response frame and provide the probe response frame to the STA.

Even if one STA in the corresponding BSS transmits the probe request forsystem information update, the probe response frame may be transmittedin a broadcast manner for system information update of other STAs in theBSS, instead of being transmitted to the one STA in a unicast mannersince the changed system information needs to be commonly applied to allSTAs in the BSS.

FIG. 24 is a diagram illustrating a probe request/response procedureaccording to another embodiment of the present invention.

The aforementioned probe response frame may include all systeminformation elements. That is, all current network information elementscan be provided to the STA irrespective of previous system informationstored in the STA. This is because it is appropriate for the systeminformation provided through the full beacon to include all systeminformation elements since the system information is for all STAs in theBSS rather than a specific STA, and the conventional probe response isappropriate when an STA does not have information regarding acorresponding network since the probe response is provided for initialassociation of the STA with the network.

However, it is more desirable to provide system information moreefficiently when an STA, which has associated with the AP and storedinformation (i.e. information before change) of the correspondingnetwork, performs operation for system information update, as proposedby the present invention. That is, since provision of the same systeminformation as system information prestored in the STA through the proberesponse frame is unnecessary and may waste resources, it is necessaryto prevent redundant provision of the system information.

Accordingly, the present invention provides provision of only a part ofcurrent system information (i.e. only one or more elements of systeminformation that needs to be updated by the STA), which has been changedfrom system information (i.e. previous system information) prestored inthe STA. A probe response frame including only information regarding asystem information change may be referred to as an optimized proberesponse frame.

Referring to FIG. 24, when a change sequence value prestored in the STAis 1 and a change sequence value included in the short beacon from theAP is 2, the STA can determine that the system information has beenchanged.

When the STA transmits the probe request frame for system informationupdate, the STA may include the change sequence value stored therein inthe probe request frame and transmit the probe request frame. Inaddition, the STA may further include information, which indicates thatthe probe request frame is a probe request frame for system informationupdate, in the probe request frame.

When the probe request frame received by the AP includes the changesequence value (or when the probe request frame includes the informationindicating that the change sequence value and the probe request frameare for system information update), the AP can compare current systeminformation with the system information (i.e. system informationcorresponding to the change sequence value stored in the STA) stored inthe STA. Upon comparison, the AP can select only a changed part of thesystem information, include the selected part in the probe responseframe and provide the probe response frame to the STA. When the APreceives the probe request frame including a change sequence value of 1in the example of FIG. 24, the AP can include, in the probe responseframe, only a current value regarding changed system informationelement(s) in a change sequence value of 2 and transmit the proberesponse frame to the STA.

FIG. 25 is a diagram illustrating a probe request/response procedureaccording to another embodiment of the present invention.

In the example of FIG. 25, the short beacon frame transmitted from theAP may be broadcast to a plurality of STAs, i.e. STA1, STA2 and STA3.Here, it is assumed that a change sequence value included in the shortbeacon frame is 5 and change sequence values corresponding to systeminformation prestored in STA1, STA2 and STA3 are 1, 2 and 2,respectively.

Accordingly, the plurality of STAs can determine that the systeminformation has been changed and transmit probe request frames includingchange sequence fields set to the values prestored therein, to the AP.

In the example of FIG. 25, the AP can transmit probe response framesincluding changed system information (i.e. system informationcorresponding to change sequence=5) in a broadcast manner upon receptionof the probe request frames. The probe response frames transmitted in abroadcast manner may include all information elements of the currentsystem information.

Alternatively, the AP may transmit the probe response frame to each ofthe plurality of STAs individually (i.e. in a unicast manner) uponreception of the probe request frames from the plurality of STAs. Inthis case, the system information included in the probe response framefor each STA may include only a part changed from the system informationstored in the corresponding STA. For example, the probe response frametransmitted to STA 1 can include only system information (i.e. currentvalue of changed information element(s) in one or more of changesequence values of 2, 3, 4 and 5) corresponding to the change sequencevalue of 5, which is changed from system information corresponding tothe change sequence value of 1. For example, the probe response frametransmitted to STA 2 or STA 3 can include only system information (i.e.current value of changed information element(s) in one or more of changesequence values of 3, 4 and 5) corresponding to the change sequencevalue of 5, which is changed from system information corresponding tothe change sequence value of 2.

Upon reception of the probe request frames for system information updatefrom the plurality of STAs, the AP can determine whether to transmit theprobe response frames in a broadcast or unicast manner, in considerationof the quantity of system information, the number of STAs that requestsystem information update, system congestion state and the like.

Embodiment 3-2

Operation similar to the system information update method using theprobe request frame/probe response frame, described in embodiment 3-1,may be performed using new request/response frames. The newrequest/response frames may be referred to as a system informationupdate request frame and a system information update response frame.Otherwise, the new request/response frames may be referred to as asystem information (SI) update request frame and an SI update responseframe. However, the scope of the present invention is not limited to thenames of the new request/response frames and includes request/responseframes in different names, used for operations provided by the presentinvention.

FIG. 26 is a diagram illustrating an SI update request/responseprocedure according to an embodiment of the present invention.

The example of FIG. 26 corresponds to the example of FIG. 25 except thatthe probe request frame is replaced by the SI update request frame andthe probe response frame is replaced by the SI update response frame,and thus redundant description is omitted.

Embodiment 3-3

FIG. 27 is a diagram illustrating a method for updating systeminformation using a full beacon request frame.

The example of FIG. 27 is discriminated from the example of FIG. 19 inthat the STA transmits the full beacon request frame in consideration ofa change sequence value included in the short beacon frame.

Specifically, when the change sequence value included in the shortbeacon frame differs from the change sequence value stored in the STA inthe example of FIG. 27, the STA can determine that the systeminformation has been changed. Accordingly, the STA can transmit the fullbeacon request frame to the AP. That is, the STA may not transmit thefull beacon request frame if the system information is not changed evenwhen the STA determines that the AP does not transmit the full beaconframe.

Upon reception of the full beacon request frame, the AP can start totransmit the full beacon frame in response to the full beacon requestframe. For example, the AP can periodically transmit the full beaconframe for a predetermined time or a predetermined number of times afterreceiving the full beacon request frame from the STA. The predeterminedtime/predetermined number of times may be set according to a valuerequested by the STA or on the basis of a value predetermined accordingto system characteristics.

Embodiment 4

As proposed in the aforementioned embodiments, the AP can transmit aresponse frame (e.g. probe response frame or SI update response frame)including a current value of information element(s) changed in thecurrent system information with reference to the change sequence valueof the STA upon reception of a request frame (e.g. probe request frameor SI update request frame) including the change sequence value of theSTA, from the STA.

To determine part of the current system information, which has beenchanged from the previous system information (e.g. system informationstored in the STA), and transmit the part, the AP needs to store systeminformation corresponding to the previous change sequence value. Here,the AP can store only the element ID of a changed information element(IE) of the system information rather than storing the changed IE of thesystem information.

Element IDs of changed IEs in system information can be provided asshown in Table 1.

TABLE 1 Information Element Element ID Inclusion of a Channel SwitchAnnouncement 37 Inclusion of an Extended Channel Switch Announcement 60Modification of the EDCA parameters 12 Inclusion of a Quiet element 40Modification of the DSSS Parameter Set 3 Modification of the CFParameter Set 4 Modification of the FH Parameter Set 8 Modification ofthe HT Operation element 45 Modification of the Channel SwitchAssignment 35 . . . . . .

When the element IDs of changed IEs are provided as shown in Table 1,change sequence values stored in the AP can be mapped to the element IDsof changed IEs according to system information change.

For example, it is assumed that EDCA parameter is changed in changesequence 1, CF parameter is changed in change sequence 2, HT operationelement is changed in change sequence 3 and EDCA parameter is changed inchange sequence 4. In this case, the AP can map the change sequencevalues to the element IDs corresponding to the changed IEs and store thechange sequence values and the element IDs. That is, the AP can store alist (referred to as a change sequence list or configuration changecount list hereinafter) regarding system information change, as shown inTable 2.

TABLE 2 Change sequence = 1 Element ID = 12 Change sequence = 2 ElementID = 4 Change sequence = 3 Element ID = 45 Change sequence = 4 ElementID = 12

As shown in Table 2, the ID of one IE can be mapped to one changesequence and stored. When change sequence information is 1 byte (i.e.information capable of representing the number of 256 cases) and elementID information mapped thereto is also 1 byte, a storage space of 2 bytesis necessary to represent one element ID mapped to one change sequence.

When it is assumed that the system information is changed according tothe aforementioned example, system information update operation can beperformed as follows.

Assuming that the STA transmits a request frame (e.g. probe requestframe or SI update request frame) including change sequence=2 and achange sequence value corresponding to the current system information ofthe network is 4, the AP can determine system information (i.e. elementID=45 and 12 in Table 2) which has been changed from system informationof change sequence of 2. Accordingly, the AP can include an HT operationelement and EDCA parameter respectively corresponding to element IDs 45and 12 in a response frame (e.g. probe response frame or SI updateresponse frame) and transmit the response frame to the STA.

As described above, the AP can store the change sequence list (orconfiguration change count (CCC) list) in which change sequence valuesare mapped to IDs of changed system information at the change sequencevalues.

When the AP maps the ID of a changed element to a change sequence valueand stores the mapped values whenever system information is changed,memory overhead of the AP may increase. For example, when changesequence information is 1 byte and element ID information is 1 byte, astorage space of 512 bytes is necessary to store element ID informationmapped to 256 different change sequence values. However, informationregarding change of old system information (i.e. change sequence valuesand element IDs mapped thereto) may be unnecessary since systeminformation is not frequently changed in general. That is, when the APmaintains a storage space of 512 bytes all the time in order to storeinformation regarding system information change, unnecessary overheadcan be generated in the memory of the AP.

Accordingly, to reduce overhead for storing information regarding systeminformation change in the AP, the number of pieces of storedinformation, that is, change sequence lists can be refreshed orrestricted according to conditions such as time, the number of changesequences and the like.

For example, the AP can limit the stored information according to time.Specifically, the AP can determine a unit of a predetermined period(e.g. a few minutes, a few hours, a few days, a few months, a few yearsor the like), retain stored information only for the correspondingperiod and delete expired information. For example, when the information(i.e. change sequence values and element ID values mapped thereto)regarding system information change is retained for month, the AP maynot retain the information regarding system information change after onemonth. In this case, the size of the storage space necessary for the APto store the information regarding system information change is notuniform. For example, while a 2-byte storage space is necessary when thesystem information has been changed once in the last month, a 20-bytestorage space is necessary when the system information has been changedten times in the last month. However, when the stored information islimited according to time, system information management stability canbe improved since previous system information is not lost even when thesystem information is frequently changed.

Alternatively, the AP may limit the stored information according to thenumber of change sequences. The number of pieces of retained informationcan be set to 4, 8, 12, 16 . . . , for example. It is assumed that theAP is configured to retain only information corresponding to latest 8change sequences and a change sequence value of current systeminformation is 16. In this case, the AP may retain change sequences of9, 10, . . . , 16 and element IDs mapped thereto but may not retain ormay delete information regarding change of the previous systeminformation (i.e. change sequences of 8, 7, 6, 5, . . . and element IDsmapped thereto). Here, the storage space necessary for the AP to storethe information regarding system information change can be maintained as16 bytes. Accordingly, system information management efficiency can beimproved.

In the aforementioned method of storing the information regarding systeminformation change, the conditions of time and the number of pieces ofstored information may be simultaneously applied. For example, systeminformation can be managed using a flexible storage space of less than20 bytes by limiting a maximum number of pieces of stored information to10 while storing information regarding system information change for thelast month.

Embodiment 5

When the STA according to the present invention has received systeminformation and change sequence information from the AP associatedtherewith through at least one of the full beacon, probe response frameand system information response frame, the STA may continuously storethe system information and change sequence information of the associatedAP even after dissociating from the AP. By storing the systeminformation and change sequence information of the dissociated AP, theSTA can perform fast initial ink setup (FILS) when re-associated withthe dissociated AP. A description will be given of examples ofperforming fast initial link setup by storing system information andchange sequence information on a dissociated AP during active scanningand passive scanning with reference to FIGS. 28 and 29.

FIG. 28 is a diagram illustrating an example of performing fast initiallink setup during active scanning.

When an STA performs active scanning for a target AP (or BSS), thetarget AP is an AP with which the STA was associated, and the STA storessystem information and change sequence information on the target AP, theSTA can configure a probe request frame such that the probe requestframe includes the change sequence information (S2801).

Upon reception of the probe request frame including the change sequenceinformation, the AP can compare current system information with systeminformation (i.e. system information corresponding to the changesequence value stored in the STA) stored in the STA. When the changesequence value received from the STA differs from a current changesequence value of the AP, the AP can include changed parts of variouspieces of system information in a probe response frame and provide theprobe response frame to the STA (S2802).

For example, when the change sequence value (=1) received from the STAcorresponds to a previous change sequence value instead of the currentchange sequence value (=2) in the change sequence list in FIG. 28, theAP can include, in the probe response frame, only a current value (i.e.current value of system information element(s), which has been changedfrom the previous change sequence (=1), in the current change sequence(=2)) of system information element(s) that needs to be updated andtransmit the probe response frame to the STA.

As described above, it is possible to reduce the size of the proberesponse frame by including only changed system information, instead ofwhole system information, in the probe response frame, thereby resultingin fast initial link setup.

When the change sequence list stored in the AP does not have a valuecorresponding to the change sequence value received from the STA, the APcannot be aware of system information that has been changed.Accordingly, the AP may configure a probe response frame including thewhole system information and the current change sequence value. Here,system information that can be included in the probe response frame maybe limited to only non-dynamic elements or to non-dynamic elements andsome dynamic elements. For a detailed description of the non-dynamicelements and dynamic elements, refer to embodiment 5-1 which will bedescribed later.

FIG. 29 is a diagram illustrating an example of performing fast initiallink setup during passive scanning.

An STA that performs passive scanning may receive a short beaconincluding change sequence information from the AP (S2910). If the AP wasassociated with the STA and system information and change sequenceinformation on the AP have been stored in the STA, then the STA maycompare the change sequence information received from the AP with thechange sequence information stored therein so as to determine whetherthe system information has a changed part. When the change sequencevalue stored in the STA is identical to the change sequence value (i.e.current change sequence value) received from the AP, the STA can beassociated with the AP using the system information stored thereinwithout receiving a full beacon.

Conversely, when the change sequence value stored in the STA differsfrom the change sequence value (i.e. current change sequence value)received from the AP, the STA can obtain system information from the APby receiving a full beacon frame at a full beacon transmission time(S2902 a), as shown in FIG. 29(a), or by receiving a probe responseframe in response to a probe request frame, as shown in FIG. 29(b).

The full beacon transmission time may be indicated by the duration tonext full beacon field included in the short beacon, as described abovewith reference to FIGS. 19 and 20, the full beacon transmission time isnot limited thereto.

When the STA receives the system information through the probe requestframe and the probe response frame, the STA can transmit the proberequest frame including the change sequence value stored therein (S2902b). When the change sequence value received from the STA differs fromthe change sequence value stored in the AP, that is, when the changesequence value received from the STA is identical to a previous changesequence value instead of the current change sequence value, the AP mayinclude, in the probe response frame, only a current value of systeminformation element(s), which have been changed from the previous changesequence (=1), in the current change sequence (=2) and transmit theprobe response frame to the STA (S2902 b). The AP may configure theprobe response frame such that the probe response frame includes thewhole system information irrespective of the change sequence value.

If a change sequence list stored in the AP does not include a valuecorresponding to the change sequence value received from the STA, thenthe AP cannot be aware of which system information has been changed.Accordingly, the AP may configure the probe response frame such that theprobe response frame includes the whole system information and thecurrent change sequence value. Here, the system information that can beincluded in the probe response frame may be limited to non-dynamicelements only or non-dynamic elements and some dynamic elements.

When the STA stores the system information and change sequenceinformation on the disassociated AP, as described above, the STA canreceive only changed system information through exchange of the proberequest frame and the probe response frame (when the stored changesequence value differs from the received change sequence value) orperform fast initial ink setup by omitting reception of the full beacon(when the stored change sequence value is identical to the receivedchange sequence value).

To this end, the STA can continuously store the system informationelement(s) and change sequence information, which have been receivedthrough the probe response frame or beacon frame (short beacon or fullbeacon) from the AP, even after the STA is dissociated from the AP.

Furthermore, the AP can store previous change sequence information andchanged system information whenever system information is changed. Here,the AP can store only the ID of a changed information element (IE)instead of the changed IE.

For example, if a channel switch assignment information element has beenchanged (added or deleted) when the change sequence value=0, then the APcan increment the change sequence value by 1, associate the changesequence value with the ID of the channel switch assignment informationelement and store the associated change sequence value and channelswitch assignment information element ID. For example, the AP can storedata such as [change sequence=1, channel switch assignment informationelement ID=35] when IDs of information elements shown in Table 1 areused. On the same principle, if an EDCA parameter set informationelement has been changed (added or deleted) when the change sequencevalue=1, then the AP can store data such as [2, 12] as a [changesequence, system information element] pair. If an HT operationinformation element has been changed (or added) when the change sequencevalue=2, then the AP can store data such as [3, 45] as a [changesequence, system information element] pair. As described above, the APcan generate and store a change sequence list (or configuration changecount list (CCC)) in which change sequence values are mapped to IDs ofsystem information changed at the corresponding change sequence values.

When the ID of a changed information element is mapped to acorresponding change sequence value and stored whenever systeminformation is changed, memory overhead of the AP may increase.Accordingly, the number of stored information elements, that is, changesequence lists may be refreshed or restricted according to conditionssuch as time, the number of information elements and the like.

Since the example of restricting the number of stored informationelements according to time or the number of information elements hasbeen described in embodiment 4, detailed description thereof is omitted.

Embodiment 5-1

Information elements of system information can be classified intotime-invariant non-dynamic elements (or fixed elements) and time-variantdynamic elements. Specifically, a timestamp, BSS load, beacon timing ofneighbor STAs, time advertisement, BSS access category (AC), BSS ACaccess delay, BSS average access delay, BSS available admission capacityand TPC report element (TPC report element can be changed twice to fivetimes per day) can correspond to dynamic elements.

Since the dynamic elements vary with time, increasing the changesequence value (or configuration change count value) due to dynamicelement change may be inefficient. Accordingly, the AP may increase thechange sequence value (or configuration change count value) only whensystem information corresponding to an element (i.e. non-dynamicelement) other than the dynamic elements has been changed.

Accordingly, the AP can compare the change sequence value transmittedfrom the STA with the change sequence value stored in the AP anddetermine whether to include a non-dynamic element in the probe responseframe. A dynamic element may be included in the probe response frame orshort beacon frame by default and transmitted.

That is, dynamic elements that cannot affect the change sequence valueare included in the probe response frame or short frame, whereasnon-dynamic elements that affect the change sequence value areselectively included in the probe response frame through comparison ofthe change sequence value stored in the STA and the change sequencevalue stored in the AP.

However, when all dynamic elements are included in the probe responseframe or short beacon frame, overhead of the probe response frame orshort beacon frame may excessively increase. Accordingly, the AP mayinclude important information (e.g. timestamp and BSS load) for APselection in the probe response frame or short beacon frame, transmitthe probe response frame or short beacon frame to the STA, andadditionally transmit the remaining dynamic elements (e.g. timeadvertisement, TPC report element and the like) to the STA in anauthentication or association process.

Alternatively, the AP may transmit all dynamic elements to the STA inthe authentication or association process without inserting any dynamicelement into the probe response frame or short beacon frame.

That is, unnecessary overhead (i.e. overhead of the short beacon frameor probe response frame) in the scanning process can be reduced bytransmitting at least part of dynamic elements to the STA throughauthentication or association so as to enable the STA to perform fastinitial link setup.

The STA may retain only non-dynamic elements other than dynamic elementswhen retaining system information of a previously associated AP. Sincethe dynamic elements vary with time, it is more desirable to obtain thedynamic elements through the beacon frame (short beacon frame or fullbeacon frame) or probe response frame received from the AP or throughauthentication or association.

Embodiment 5-2

The STA may store system parameter(s) and configuration change countvalue (or change sequence value) of only a preferred AP from amongprevious associated APs. In this case, the AP can maintain anappropriate storage space for storing information regarding systeminformation change to improve system information management efficiency.

To set an associated AP as a preferred AP, the STA may request the AP toset the STA as a preferred STA. FIG. 30 is a diagram illustrating aprocedure of setting an associated AP as a preferred AP. The STA canrequest the associated AP to set the STA as a preferred STA. Here, theSTA may request the associated AP to set the STA as a preferred STA bytransmitting an existing request frame (e.g. association request frame)or a new request frame (e.g. short probe request frame, optimized proberequest frame, FILS probe request frame, preferred STA request frame orthe like) to the AP after link setup (i.e. scanning, authentication andassociation), as shown in FIG. 30.

Alternatively, the STA may request the associated AP to set the STA as apreferred STA by transmitting an existing request frame or a new requestframe that includes a field indicating the request during the link setupprocedure.

Upon reception of the request from the STA, the AP may reject or acceptthe request of the STA. When the AP rejects the request of the STA, theAP may notify the STA of rejection of the request of the STA bytransmitting an existing response frame (e.g. association responseframe) or a new response frame (e.g. short probe response frame,optimized probe response frame, FILS probe response frame, preferred STAresponse frame or the like). For example, when many STAs are registeredas preferred STAs, the AP can reject the request of the STA. When the APrejects the request of the STA, the AP may delete information of the STA(e.g. capability of the STA) when the STA is de-associated from the AP(refer to FIG. 30(a)).

When the AP accepts the request of the STA, the AP may store systeminformation of the STA and system information about capability of theSTA and notify the STA of acceptance of the request of the STA bytransmitting an existing response frame or a new response frame. Whenthe AP accepts the request of the STA, the AP can retain the informationof the STA (e.g. capability of the STA) even if the STA is de-associatedfrom the AP (refer to FIG. 30(b)).

Upon reception of the response frame indicating that the request of theSTA has been accepted, the STA may set the AP as a preferred AP. Then,the STA may store system parameter(s) and a configuration change countvalue (or change sequence value) regarding the preferred AP even whenthe STA is disassociated from the AP.

When the STA stores the system parameter(s) regarding the preferred AP,the STA may include AP configuration change count information (or changesequence information) on the AP in the probe request frame and transmitthe probe request frame to the AP when attempting active scanning forthe preferred AP (or target AP).

FIG. 31 is a diagram illustrating operation when active scanning isperformed on a previously de-associated preferred AP. As shown in FIG.31, when the STA attempts active scanning for a preferred AP, the STAmay include AP configuration change count information (or changesequence information) on the AP in the probe request frame and transmitthe probe request frame to the AP (S3101).

Upon reception of the probe request frame including the configurationchange count information from the STA, the AP may compare the receivedconfiguration change count value with a current configuration changecount value and configure a probe response frame on the basis of thecomparison result (S3102).

For example, when the received configuration change count value isidentical to the current configuration change count value, the AP canexclude an optional information element and include, in the proberesponse frame, mandatory field(s) (e.g. timestamp, capability andbeacon interval) or elements (i.e. frequency varying informationelements (e.g. dynamic elements of system information) which areirrelevant to the change sequence value, along with the current APconfiguration change count value (identical to the configuration changecount value stored in the STA) and transmit the probe response frameincluding the mandatory field(s) or element and the current APconfiguration change count value to the STA.

When the received configuration change count value differs from thecurrent configuration change count value but corresponds to a previousconfiguration change count value, the AP may determine that a changedsystem parameter needs to be transmitted, include mandatory field(s) andan optional information element (i.e. the changed system parameter) inthe probe response frame and transmit the probe response frame to theSTA.

When a configuration change count list stored in the AP does not includethe configuration change count value received from the STA, the APcannot be aware of which system information has been changed.Accordingly, the AP may configure the probe response frame such that theprobe response frame includes the whole system information and thecurrent change sequence value. Here, system information that can beincluded in the probe response frame may be limited to non-dynamicelements only or the non-dynamic elements and some dynamic elements.

When the AP determines that the changed system parameter need not betransmitted to the STA even though the received configuration changecount value differs from the current configuration change count value,the AP may exclude an optional information element and configure theprobe response frame such that the probe response frame includesmandatory field(s) and the current AP configuration change count value.

Embodiment 5-3

As described above with reference to FIG. 5 and the related embodiments,when the STA performs active scanning for the preferred AP, theoptimized probe request frame including only information regardingsystem information change can be used instead of the normal proberequest frame.

The optimized probe request frame may be called a short probe requestframe, FILS probe request frame or the like since the optimized proberequest frame uses a smaller quantity of information than the normalprobe request frame (the FILS probe request frame is used as arepresentative example in the present embodiment).

The FILS probe request frame can include one of the followinginformation.

i) STA address (MAC address): An STA that performs active scanning caninclude the MAC address thereon in the FILS probe request frame.

ii) BSSID or partial BSSID: Since the STA knows address information of apreferred AP, the STA can include the corresponding BSSID or partialBSSID in the MAC PDU of the FILS probe request frame.

iii) Configuration change count information (or change sequenceinformation) of a preferred AP: Configuration change count informationindicates whether system information of the AP has been changed. The STAcan store (retain) a configuration change count value, which wasreceived from a previously associated preferred AP, even afterde-association from the preferred AP and include the storedconfiguration change count value in the FILS probe request frame whenperforming active scanning for the preferred AP.

iv) STA capability which was transmitted through the probe request frameor optional information element(s) related to system information: WhenSTA capability or optional information element(s) have been changed, theSTA needs to notify the AP that the capability or optional informationelement(s) have been changed. Accordingly, when STA capability oroptional information element(s) have been changed after de-associationof the STA from the preferred AP, the changed information can beincluded in the FILS probe request frame.

However, the FILS probe request frame may not include the STA capabilityor optional information element(s) since the STA capability is notchanged in general.

The FILS probe request frame will now be described in more detail withreference to the attached drawings.

FIG. 32 illustrates an exemplary FILS probe request frame. Referring toFIG. 32, the FILS frame request frame may include MAC header, proberequest body and FCS fields.

The address (MAC address) of the STA and BSSID (or partial BSSID) may beincluded in the MAC header.

The MAC header is 36 bytes and the FCS is 4 bytes. When 1-byteconfiguration change count information is included in the form of aninformation element in the probe request body, a 2-byte (element IDfield (1 byte) and length field (1 byte) of the configuration changecount field) overhead is added. Accordingly, a total overhead of theFILS probe request frame for including the 1-byte configuration changecount information may be 42 bytes and the size of the MAC PDU of theFILS probe request frame including no optional information element(s)may be 43 bytes.

If the configuration change count value is always included in the FILSprobe request frame as a default value instead of an informationelement, then the MAC PDU of the FILS probe request frame will become 41bytes.

To further reduce the overhead of the FILS probe request frame, a shortMAC header may be used instead of the MAC header. FIG. 33 illustrates anexemplary FILS probe request frame to which the short MAC header isapplied. Referring to FIG. 33, the FILS frame request frame may includeshort MAC header, probe request body and FCS fields. When the short MACheader is used, the overhead of the FILS probe request frame can befurther decreased. FIG. 34 illustrates an example of the short MACheader.

FIG. 34 illustrates the short MAC header. Referring to FIG. 34, theshort MAC header includes a frame control (FC) field, an AID field, aBSSID (or receiver address (RA)) field and a sequence control field andmay selectively include an A3 field.

Sub-fields of the frame control field are shown in FIG. 34(b). The framecontrol field can indicate whether the MAC header is the short MACheader. Further, the frame control field can indicate whether the shortMAC header includes the A3 field.

The positions of the AID field and BSSID field may be controlledaccording to a value of From-distribution system (DS) included in the FCfield. Since the short probe request frame is transmitted to an AP inthe same BSS to which the STA belongs, in general, From-DS will be setto “0”. Accordingly, the BSSID field is disposed in A1 following the FCfield and the AID of the STA is included in A2 in general. However, thepositions are not limited thereto.

The short MAC header may further include a sequence control field.Sub-fields of the sequence control field are shown in FIG. 34(c).

When the short MAC header shown in FIG. 34 is used, the size of theshort MAC header is 12 bytes, the size of the FCS field is 4 bytes, anda 14-byte overhead for including a 1-byte configuration change countvalue, including a 2-byte information element overhead of theconfiguration change count information, is generated in order toincluding a 1-byte configuration change count value, including a 2-byteinformation element overhead of the configuration change countinformation, is generated. The size of the MAC PDU of the FILS proberequest frame can be 19 bytes. If the configuration change countinformation is included as a default value instead of an informationelement and optional information element(s) is not included, then theMAC PDU of the FILS probe request frame becomes 17 bytes.

The format of the short MAC header is not limited to the example of FIG.34. FIG. 35 illustrates another exemplary MAC header. As shown in FIG.35, the short MAC header may include a frame control field, adestination MAC address field, a source MAC address field, a sequencecontrol field, a body field and an FCS field.

The destination MAC address field may include the BSSID (or partialBSSID) of the corresponding AP and the source MAC address field mayinclude the MAC address of the corresponding STA. Whether the MAC headeris the short MAC header can be indicated through the frame controlfield.

When the short MAC header shown in FIG. 35 is used, the size of theshort MAC header is 16 bytes, the size of the FCS field is 4 bytes, anda 22-byte overhead including a 2-byte information element overheadregarding the configuration change count value is generated. The size ofthe MAC PDU of the FILS probe request frame can be 23 bytes. If theconfiguration change count value is included as a default value insteadof an information element and optional information element(s) are notincluded, then the MAC PDU of the FILS probe request frame becomes 21bytes.

The FILS probe request frame may be defined differently from that shownin FIG. 32. FIG. 36 illustrates another example of the FILS proberequest frame. Referring to FIG. 36, the FILS probe request frame mayinclude a frame control (FC) field, a destination address (DA) field, asource address (SA) field, a change sequence (or configuration changecount) field, optional information element(s) and an FCS field.

Whether the probe request frame is the FILS probe request frame can beindicated through the FC field, specifically, type and sub-type fieldsof the FC field. For example, type=11 and sub-type=0010 can indicatethat the probe request frame is the FILS probe request frame. Whetherthe probe request frame is the FILS probe request frame may be indicatedusing methods other than the type and sub-type fields.

The DA field may be set to the BSSID (or partial BSSID) and the SA fieldmay be set to the MAC address of the STA.

When the FILS probe request frame shown in FIG. 36 is used, the MPDU ofthe FILS probe request frame can have a size of 13 bytes.

As described above with reference to FIG. 5 and the related embodiments,the AP can use the optimized probe response frame including onlyinformation that needs to be changed when transmitting systeminformation to the STA. Since the optimized probe response frameincludes a smaller quantity of information than the normal proberesponse frame, the optimized probe response frame may be called a shortprobe response frame, an FILS probe response frame or the like (the FILSprobe response frame is used as a representative in the presentembodiment).

FIG. 37 illustrates an example of the FILS probe response frame. Asshown in FIG. 37, the FILS probe response frame may include a framecontrol field, a destination address field, a source address field, atimestamp field, a change sequence field (or configuration change countfield), optional information element field and an FCS field.

Whether the probe response frame is the FILS probe response frame can beindicated through the frame control field.

The destination address field may include the MAC address of thecorresponding STA and the source address field may include the BSSID (orpartial BSSID) of the corresponding AP.

Since the timestamp is dynamic system information varying in real time,whether the timestamp has been changed is not indicated by theconfiguration change count. The STA can always obtain a timestamp valuethrough the timestamp field of the FILS probe request frame irrespectiveof whether the configuration change count value has been changed.

The configuration change count field may include a change sequence value(or configuration change count value), which was obtained by the STAfrom a preferred AP when the STA was associated with the AP. While theconfiguration change count field may be included as a default value inthe FILS probe response frame, as shown in FIG. 37, the configurationchange count field may be included in the form of an information element(i.e. addition of the element ID field and length field of the changesequence field).

The optional information element field may include information elementsof system information that needs to be updated by the STA. Furthermore,dynamic elements other than the timestamp, that is, system informationthat does not affect the configuration change count value, can beincluded in the optional information element field if the dynamicelements are supported by the AP. Specifically, the optional informationelement field may include a BSS load, beacon timing of neighbor STAs,time advertisement, BSS access category (AC), BSS AC access delay, BSSaverage access delay, BSS available admission capacity and TPC reportelement) (the TPC report element can be changed twice to five times aday) according to whether the AP supports the elements.

FIG. 38 is a diagram illustrating a system information updaterequest/response procedure according to an embodiment of the presentinvention.

The example of FIG. 38 is identical to the example of FIG. 31 exceptthat the probe request frame is replaced by the FILS probe request frameand the probe response frame is replaced by the FILS probe responseframe, and thus description thereof is omitted.

Embodiment 5-4

An operation similar to the aforementioned system information updatemethod using the probe request frame/probe response frame may beperformed using new request/response frames, which are different fromthose described in embodiment 5-4. The new request/response frames canbe referred to as system information update request/response frames.Otherwise, the new request/response frames may be referred to as systeminformation (SI) update request/response frames. However, the scope ofthe present invention is not limited to the names of the newrequest/response frames and includes request/response frames indifferent names, which are used in operations proposed by the presentinvention.

The new request/response frames may have a null-data packet (NDP) frameformat.

Embodiment 5-5

When the AP receives probe request frames including configuration changecount values from one or more STAs, the AP may compare the receivedconfiguration change count values with a current configuration changecount value and then unicast an appropriately configured probe responseframe to an STA that needs system information update.

FIG. 39 illustrates an example of unicasting the probe response frame.As shown in FIG. 39, when the current configuration change count valueof the AP is 6 whereas configuration change count values received fromSTA 1, STA 2 and STA 3 are respectively 3, 4 and 5, the AP can unicast aprobe response frame including system information corresponding toconfiguration change counts 4, 5 and 6 to STA 1, unicast a proberesponse frame including system information corresponding toconfiguration change counts 5 and 6 to STA 2 and unicast a proberesponse frame including system information corresponding toconfiguration change 6 to STA 3.

In the example shown in FIG. 39, however, the AP needs to transmit asmany probe response frames as the number of STAs that have transmittedprobe request frames even though redundant information (STAs 1, 2 and 3need to commonly receive system information corresponding toconfiguration change count 6) is present.

Accordingly, the AP may include system information elements that need tobe updated by respective STAs in one probe response frame and thenbroadcast the probe response frame to the STAs.

FIG. 40 illustrates an example of broadcasting a probe response frame.When the current configuration change count value of the AP is 6 whereasconfiguration change count values received from STA 1, STA 2 and STA 3are respectively 3, 4 and 5, as in the example shown in FIG. 40, the APcan determine system information that needs to be updated by the STAs onthe basis of the lowest configuration change count value. Since theconfiguration change count value received from STA 1 is the smallest inthe example of FIG. 40, the AP can determine system information canconfigure a probe response frame including system informationcorresponding to configuration change counts 4, 5 and 6 on the basis ofthe configuration count value received from STA 1 and broadcast theprobe response frame.

STAs 1, 2 and 3 can receive the broadcast probe response frame andupdate the system information.

Embodiment 5-6

In some aforementioned embodiments, the STA recognizes the currentchange sequence value (or configuration change count value) of the AP byreceiving a short beacon from the AP. Alternatively, the change sequencevalue (or configuration change count value) of the AP may be transmittedto the STA through an FILS discovery frame.

The FILS discovery frame supports a quick AP (or network) for fastinitial link setup and can be transmitted by an STA (i.e. AP) thattransmits a beacon frame.

Embodiment 6

Even though the STA stores configuration change count information andsystem information on a preferred AP even after disassociation from thepreferred AP, information (i.e. capability of the STA) on the STA andconfiguration change count information may be deleted from the AP if theAP is restarted due to reset or power outrage of the AP. However, theSTA cannot correctly receive system information even if the STA comparesconfiguration change count information since the STA cannot be aware ofwhether the preferred AP is restarted.

To solve this problem, when the restarted AP receives an FILS proberequest frame from the preferred STA, the AP can include the duration tonext full beacon field, information on the next TBTT or information forrequesting a normal probe request frame, in an FILS probe response framesuch that the STA can correctly receive the system information.

FIG. 41 illustrates an example in which an FILS response frame includingthe duration to next full beacon field or information on the next TBTT.Upon reception of an FILS probe request frame including change sequenceinformation (or configuration change count information) from thepreferred STA after restarted, the AP can transmit the FILS proberesponse frame including information on the next TBTT or the duration tonext full beacon field to the STA.

The STA can receive a full beacon at a full beacon transmission timeindicated by the FILS probe response frame and update systeminformation.

FIG. 42 illustrates an example in which an FILS response frame includesinformation for requesting transmission of a normal probe request frame.Upon reception of an FILS probe request frame including change sequenceinformation (or configuration change count information) from thepreferred STA after restart, the AP can transmit the FILS probe responseframe including the information for requesting transmission of a normalprobe request frame, as shown in FIG. 42. Upon reception of the FILSprobe response frame, the STA can transmit the normal probe requestframe, receive a normal probe response frame as a response to the normalprobe request frame from the AP and update system information.

Which one of the information on the next TBTT (or duration to next fullbeacon field) and the information for requesting transmission of thenormal probe request frame is included in the FILS response frame can bedetermined according to a duration to a transmission time of the nextbeacon (i.e. next TBTT). When the STA can immediately receive a fullbeacon since the next TBTT is short, the AP can include the informationon the next TBTT (or duration to next full beacon field) in the FILSresponse frame. When the STA cannot receive a full beacon for a whilesince the next TBTT is long, the AP can include the information forrequesting transmission of the normal probe request frame in the FILSresponse frame so as to support fast initial ink setup.

In the aforementioned system information update method according to thepresent invention, the above described various embodiments of thepresent invention may be independently applied or two or more thereofmay be simultaneously applied.

FIG. 43 is a block diagram illustrating a configuration of a radioapparatus according to an embodiment of the present invention.

An AP 10 may include a processor 11, a memory 12 and a transceiver 13.An STA 20 may include a processor 21, a memory 22 and a transceiver 23.The transceivers 13 and 23 may transmit/receive radio signals andimplement a physical layer according to IEEE 802, for example. Theprocessors 11 and 21 may be connected to the transceivers 13 and 23 andimplement the physical layer and/or a MAC layer according to IEEE 802.The processors 11 and 21 may be configured to perform operationsaccording to the aforementioned various embodiments of the presentinvention. Further, modules that implement operations of the AP and STAaccording to the aforementioned various embodiments of the presentinvention may be stored in the memories 12 and 22 and executed by theprocessors 11 and 21. The memories 12 and 22 may be included in theprocessors 11 and 21 or provided to the outside of the processors 11 and21 and connected to the processors 11 and 21 using a known means.

Detailed configurations of the aforementioned AP and STA may beimplemented such that the above-described various embodiments of thepresent invention can be independently applied or two or more thereofcan be simultaneously applied, and redundant description is omitted forclarity.

The embodiments of the present invention may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof.

In a hardware configuration, an embodiment of the present invention maybe achieved by one or more ASICs (application specific integratedcircuits), DSPs (digital signal processors), DSPDs (digital signalprocessing devices), PLDs (programmable logic devices), FPGAs (fieldprogrammable gate arrays), processors, controllers, microcontrollers,microprocessors, etc.

In a firmware or software configuration, an embodiment of the presentinvention may be implemented in the form of a module, a procedure, afunction, etc. Software code may be stored in a memory unit and executedby a processor. The memory unit is located at the interior or exteriorof the processor and may transmit and receive data to and from theprocessor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

INDUSTRIAL APPLICABILITY

While various embodiments of the present invention have been describedon the basis of IEEE 802/11, the embodiments can be equally applied tovarious mobile communication systems.

The invention claimed is:
 1. A method for updating system information ina station (STA) of a wireless communication system, comprising:transmitting, via the STA, a probe request frame, including aconfiguration change count field, to a preferred access point (AP); andreceiving, via the STA, a probe response frame from the preferred AP,wherein if a value of the configuration change count field included inthe probe request frame is different from a current count value of thepreferred AP, the probe response frame includes one or more elements ofsystem information needed to be updated to the STA, wherein a countvalue of the preferred AP increases when a non-dynamic element of thesystem information is updated, and wherein the count value of thepreferred AP is associated with the value of the configuration changecount field.
 2. The method according to claim 1, wherein the count valueof the preferred AP is not increased when a dynamic element of thesystem information is updated, and the dynamic element of the systeminformation includes at least one of a timestamp, basic service set(BSS), BSS load, beacon timing, time advertisement, BSS access category(AC), BSS AC access delay, BSS average access delay, BSS availableadmission capacity or TPC report element.
 3. The method according toclaim 1, wherein a length of the configuration change count field isdefined as a 1-octet field and the value of the configuration changecount field sets in a range of 0 to
 255. 4. The method according toclaim 1, wherein the probe response frame mandatorily comprises at leastone of a timestamp field, a capability field or a beacon interval field.5. The method according to claim 1, wherein the probe response frame isbroadcasted when the preferred AP receives the probe request frame froma plurality of STAs.
 6. The method according to claim 1, wherein thevalue of the configuration change count field corresponds to a pastcount value of the preferred AP which is obtained when the STApreviously associated with the preferred AP.
 7. The method according toclaim 6, wherein if the preferred AP is restarted before receiving theprobe request frame after previous disassociation with the STA, theprobe response frame includes information indicating a next beaconreception time.
 8. A method for providing system information updated inan access point (AP) of a wireless communication system, comprising:receiving, via the AP, a probe request frame, including a configurationchange count field; and transmitting, via the AP, a probe response frameto a preferred STA, wherein if a value of the configuration change countfield included in the probe request frame is different from a currentcount value of the AP, the probe response frame includes one or moreelements of system information needed to be updated to the preferredSTA, wherein a count value of the AP increases when a non-dynamicelement of the system information is updated, and wherein the countvalue of the AP is associated with the value of the configuration changecount field.
 9. The method according to claim 8, wherein the count valueof the AP is not increased when a dynamic element of the systeminformation is updated, and the dynamic elements of the systeminformation includes at least one of a timestamp, basic service set(BSS), BSS load, beacon timing, time advertisement, BSS access category(AC), BSS AC access delay, BSS average access delay, BSS availableadmission capacity or TPC report element.
 10. The method according toclaim 8, wherein the probe response frame mandatorily comprises at leastone of a timestamp field, a capability field or a beacon interval field.11. A station (STA) configured to update system information in awireless communication system, comprising: a transceiver; and aprocessor, wherein the processor is configured to control thetransceiver to transmit a probe request frame, including a configurationchange count field, to a preferred AP and to receive a probe responseframe from the preferred AP, wherein if a value of the configurationchange count field included in the probe request frame is different froma current count value of the preferred AP, the probe response frameincludes one or more elements of system information needed to be updatedto the STA, wherein a count value of the preferred AP increases when anon-dynamic element of the system information is updated, and whereinthe count value of the preferred AP is associated with the value of theconfiguration change count field.
 12. An access point (AP) configured toprovide updated system information in a wireless communication system,comprising: a transceiver; and a processor, wherein the processor isconfigured to control the transceiver to receive a probe request frameincluding a configuration change count field from a station (STA) and totransmit a probe response frame to the STA, wherein if a value of theconfiguration change count field included in the probe request frame isdifferent from a current count value of the AP, the probe response frameincludes one or more elements of system information needed to be updatedto the STA, wherein a count value of the AP increases when a non-dynamicelement of the system information is updated, and wherein the countvalue of the AP is associated with the value of the configuration changecount field.